Add platform to Gemfile to skip some gems on JRuby.

I'm having trouble installing redcarpet on JRuby.
Make it pending on JRuby, too.

On JRuby we are getting a failure on travis that I can’t repro locally:

  1) RSpec behaves like a library that issues no warnings when loaded issues no warnings when loaded
     Failure/Error: expect(stderr).to eq("")
       
       expected: ""
            got: "RuntimeError: No non-lib lines in stack\n  first_non_rspec_line at /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-support-39cf8df8b572/lib/rspec/support/caller_filter.rb:46\n             deprecate at /home/travis/build/rspec/rspec-core/lib/rspec/core/warnings.rb:11\n               autorun at /home/travis/build/rspec/rspec-core/lib/rspec/core/runner.rb:15\n                (root) at /home/travis/build/rspec/rspec-core/lib/rspec/autorun.rb:2\n               require at org/jruby/RubyKernel.java:1065\n                (root) at /home/travis/.rvm/rubies/jruby-1.7.13-d19/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:1\n               require at /home/travis/.rvm/rubies/jruby-1.7.13-d19/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:55\n"
       
       (compared using ==)
       
       Diff:
       @@ -1 +1,9 @@
       +RuntimeError: No non-lib lines in stack
       +  first_non_rspec_line at /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-support-39cf8df8b572/lib/rspec/support/caller_filter.rb:46
       +             deprecate at /home/travis/build/rspec/rspec-core/lib/rspec/core/warnings.rb:11
       +               autorun at /home/travis/build/rspec/rspec-core/lib/rspec/core/runner.rb:15
       +                (root) at /home/travis/build/rspec/rspec-core/lib/rspec/autorun.rb:2
       +               require at org/jruby/RubyKernel.java:1065
       +                (root) at /home/travis/.rvm/rubies/jruby-1.7.13-d19/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:1
       +               require at /home/travis/.rvm/rubies/jruby-1.7.13-d19/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:55
     Shared Example Group: "a library that issues no warnings when loaded" called from ./spec/rspec/core_spec.rb:9
     # /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-expectations-62efbf65a920/lib/rspec/expectations/fail_with.rb:30:in `fail_with'
     # /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-expectations-62efbf65a920/lib/rspec/expectations/handler.rb:35:in `handle_failure'
     # /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-expectations-62efbf65a920/lib/rspec/expectations/handler.rb:48:in `handle_matcher'
     # /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-expectations-62efbf65a920/lib/rspec/expectations/expectation_target.rb:54:in `to'
     # /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-support-39cf8df8b572/lib/rspec/support/spec/prevent_load_time_warnings.rb:37:in `(root)'
     # org/jruby/RubyBasicObject.java:1562:in `instance_exec'
     # ./lib/rspec/core/example.rb:148:in `run'
     # org/jruby/RubyProc.java:271:in `call'
     # ./lib/rspec/core/example.rb:211:in `call'
     # ./spec/spec_helper.rb:127:in `(root)'
     # org/jruby/RubyBasicObject.java:1562:in `instance_exec'
     # ./lib/rspec/core/example.rb:301:in `instance_exec'
     # ./lib/rspec/core/hooks.rb:430:in `run'
     # org/jruby/RubyProc.java:271:in `call'
     # ./lib/rspec/core/example.rb:211:in `call'
     # ./spec/spec_helper.rb:124:in `(root)'
     # org/jruby/RubyBasicObject.java:1562:in `instance_exec'
     # ./spec/spec_helper.rb:82:in `sandboxed'
     # /home/travis/build/rspec/bundle/jruby/1.9/bundler/gems/rspec-mocks-67644b79082a/lib/rspec/mocks.rb:94:in `with_temporary_scope'
     # ./spec/spec_helper.rb:81:in `sandboxed'
     # ./spec/spec_helper.rb:124:in `(root)'
     # org/jruby/RubyBasicObject.java:1562:in `instance_exec'
     # ./lib/rspec/core/example.rb:301:in `instance_exec'
     # ./lib/rspec/core/hooks.rb:430:in `run'
     # org/jruby/RubyProc.java:271:in `call'
     # ./lib/rspec/core/example.rb:211:in `call'
     # ./lib/rspec/core/hooks.rb:428:in `run'
     # ./lib/rspec/core/hooks.rb:485:in `run'
     # ./lib/rspec/core/example.rb:310:in `with_around_example_hooks'
     # ./lib/rspec/core/example.rb:145:in `run'
     # ./lib/rspec/core/example_group.rb:496:in `run_examples'
     # org/jruby/RubyArray.java:2412:in `map'
     # ./lib/rspec/core/example_group.rb:492:in `run_examples'
     # ./lib/rspec/core/example_group.rb:459:in `run'