Parse coffee script stack traces

Coffeescript stack traces were parsing incorrectly. Before this change, for a stack trace of:

`at example (/tmp/example.coffee:2:3, <js>:5:20)`

rollbar_node thought the *filename* was:

`/tmp/example.coffee:2:3, <js>`

I'm now stripping out the <js> part of the stacktrace entirely. It's unfortunate to lose that information, but I'm not sure if there's a way to avoid that.