Notification: be more concrete in what file to skip

The old regexp skips even files unrelated to the Bugsnag Ruby project.
Namely, it was skipping stack traces for my bugsnag-demo project.
The new regexp ensures that it skips only Bugsnag Ruby files.
Merge pull request #132 from kyrylo/stacktraces-skipping-fix

Notification: be more concrete in what file to skip