Internal backtrace

I have added a failing test case at: https://github.com/vangberg/raven-ruby/compare/master...internal-backtrace

These kinds of backtraces come from e.g. Passenger:

```
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/abstract_server_collection.rb:82:in `block in synchronize'
<internal:prelude>:10:in `synchronize'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/abstract_server_collection.rb:79:in `synchronize'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/spawn_manager.rb:244:in `spawn_rack_application'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/spawn_manager.rb:137:in `spawn_application'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/spawn_manager.rb:275:in `handle_spawn_application'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
/usr/local/ruby_1.9.3-p194/lib/ruby/gems/1.9.1/gems/passenger-3.0.15/helper-scripts/passenger-spawn-server:99:in `<main>'
```

The regexp might have to be more strict in the matching.