Handle exception message with invalid UTF-8; For rbx

See https://travis-ci.org/mikel/mail/jobs/33707769#L129

```plain
An exception occurred running /home/travis/build/mikel/mail/gemfiles/vendor/bundle/rbx/2.1/gems/rspec-core-3.0.4/exe/rspec:
    invalid byte sequence in UTF-8 (ArgumentError)
Backtrace:
    Rubinius::Splitter.valid_encoding? at kernel/common/splitter.rb:17
    Rubinius::Splitter.split at kernel/common/splitter.rb:22
    String#split at kernel/common/string.rb:515
    RSpec::Core::Notifications::FailedExampleNotification#failure_lines at \
          gemfiles/vendor/bundle/rbx/2.1/gems/rspec-core-3.0.4/lib/rspec/core/notifications.rb:220
```

To reproduce with just the one test

```sh
 rspec spec/rspec/core/formatters/base_text_formatter_spec.rb:108
```
I understand the formatter failures from that build.  The one I was specifically wondering about were the DRb specs.  Here's the failure from that build:

```
   4) RSpec::Core::DRbRunner with server running outputs colorized text when running with --color option
     Failure/Error: expect(out.string).to include(failure_symbol)
       expected "" to include "\e[31mF"
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-expectations-b12657c551d9/lib/rspec/expectations/fail_with.rb:30:in `fail_with'
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-expectations-b12657c551d9/lib/rspec/expectations/handler.rb:35:in `handle_failure'
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-expectations-b12657c551d9/lib/rspec/expectations/handler.rb:48:in `handle_matcher'
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-expectations-b12657c551d9/lib/rspec/expectations/expectation_target.rb:54:in `to'
     # ./spec/rspec/core/drb_spec.rb:100:in `__script__'
     # kernel/common/eval.rb:101:in `instance_exec'
     # ./lib/rspec/core/example.rb:166:in `run'
     # kernel/bootstrap/proc.rb:20:in `call'
     # ./lib/rspec/core/example.rb:236:in `call'
     # ./spec/support/isolated_home_directory.rb:10:in `__script__'
     # /home/travis/.rvm/rubies/rbx/gems/gems/rubysl-tmpdir-2.0.1/lib/rubysl/tmpdir/tmpdir.rb:83:in `mktmpdir'
     # ./spec/support/isolated_home_directory.rb:6:in `__script__'
     # kernel/common/eval.rb:101:in `instance_exec'
     # ./lib/rspec/core/example.rb:337:in `instance_exec'
     # ./lib/rspec/core/hooks.rb:387:in `execute_with'
     # ./lib/rspec/core/hooks.rb:454:in `run'
     # kernel/bootstrap/proc.rb:20:in `call'
     # ./lib/rspec/core/example.rb:236:in `call'
     # kernel/common/method.rb:71:in `call'
     # kernel/common/proc.rb:172:in `call (__yield__)'
     # kernel/common/dir.rb:128:in `chdir'
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-support-3b48ab54b732/lib/rspec/support/spec/with_isolated_directory.rb:6:in `__script__'
     # /home/travis/.rvm/rubies/rbx/gems/gems/rubysl-tmpdir-2.0.1/lib/rubysl/tmpdir/tmpdir.rb:83:in `mktmpdir'
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-support-3b48ab54b732/lib/rspec/support/spec/with_isolated_directory.rb:5:in `__script__'
     # kernel/common/eval.rb:101:in `instance_exec'
     # ./lib/rspec/core/example.rb:337:in `instance_exec'
     # ./lib/rspec/core/hooks.rb:387:in `execute_with'
     # ./lib/rspec/core/hooks.rb:454:in `run'
     # kernel/bootstrap/proc.rb:20:in `call'
     # ./lib/rspec/core/example.rb:236:in `run (call)'
     # ./spec/support/sandboxing.rb:2:in `__script__'
     # kernel/common/eval.rb:101:in `instance_exec'
     # ./spec/support/sandboxing.rb:36:in `sandboxed'
     # /home/travis/build/rspec/bundle/rbx/2.1/bundler/gems/rspec-mocks-5254af27054a/lib/rspec/mocks.rb:94:in `with_temporary_scope'
     # ./spec/support/sandboxing.rb:35:in `sandboxed'
     # ./spec/support/sandboxing.rb:2:in `__script__'
     # kernel/common/eval.rb:101:in `instance_exec'
     # ./lib/rspec/core/example.rb:337:in `instance_exec'
     # ./lib/rspec/core/hooks.rb:387:in `execute_with'
     # ./lib/rspec/core/hooks.rb:454:in `run'
     # kernel/bootstrap/proc.rb:20:in `call'
     # ./lib/rspec/core/example.rb:236:in `call'
     # ./lib/rspec/core/hooks.rb:453:in `run'
     # ./lib/rspec/core/hooks.rb:509:in `run'
     # ./lib/rspec/core/example.rb:346:in `with_around_example_hooks'
     # ./lib/rspec/core/example.rb:163:in `run'
     # ./lib/rspec/core/example_group.rb:513:in `run_examples'
     # kernel/bootstrap/array.rb:97:in `map'
     # ./lib/rspec/core/example_group.rb:509:in `run_examples'
     # ./lib/rspec/core/example_group.rb:475:in `run'
     # ./lib/rspec/core/example_group.rb:476:in `run'
     # kernel/bootstrap/array.rb:97:in `map'
     # ./lib/rspec/core/example_group.rb:476:in `run'
     # ./lib/rspec/core/runner.rb:112:in `run_specs'
     # kernel/bootstrap/array.rb:97:in `map'
     # ./lib/rspec/core/runner.rb:112:in `run_specs'
     # ./lib/rspec/core/reporter.rb:62:in `report'
     # ./lib/rspec/core/runner.rb:108:in `run_specs'
     # ./lib/rspec/core/runner.rb:86:in `run'
     # ./lib/rspec/core/runner.rb:70:in `run'
     # ./lib/rspec/core/runner.rb:38:in `invoke'
     # ./exe/rspec:4:in `__script__'
     # kernel/common/kernel.rb:465:in `load'
     # script/rspec_with_simplecov:42:in `__script__'
     # kernel/delta/code_loader.rb:66:in `load_script'
     # kernel/delta/code_loader.rb:152:in `load_script'
     # kernel/loader.rb:645:in `script'
     # kernel/loader.rb:799:in `main'
```

That doesn't look backtrace related at all. It looks like on RBx that the colorization doesn't work when using DRb.