Filter bundle related lines from backtraces.

This cleans the backtraces shown for failing examples. Works with both RSpec 2 and RSpec 3 (and should degrade gracefully for RSpec 1).

Before:
```
./spec/controllers/people_controller_spec.rb:18 :in `block (3 levels) in <top (required)>'
/Users/stefan/Library/Application Support/Avian/Bundles/RSpec.tmbundle/Support/lib/rspec/mate/runner.rb:61 :in `block in run'
/Users/stefan/Library/Application Support/Avian/Bundles/RSpec.tmbundle/Support/lib/rspec/mate/runner.rb:58 :in `chdir'
/Users/stefan/Library/Application Support/Avian/Bundles/RSpec.tmbundle/Support/lib/rspec/mate/runner.rb:58 :in `run'
/Users/stefan/Library/Application Support/Avian/Bundles/RSpec.tmbundle/Support/lib/rspec/mate/runner.rb:18 :in `run_file'
/tmp/textmate-command-60237.rb:3 :in `<main>'
```

After:
```
./spec/controllers/people_controller_spec.rb:18 :in `block (3 levels) in <top (required)>'
```