Generalize the gem backtrace exclusion.

Previous regex matcher let gems slip through if a custom path was
provided to bundler (e.g. `bundle install --path ../bundle`). Remove any
explicit path dependencies on potential ruby version based directory
names.
So it looks like previously `ruby` was in the path but it's not any more?  That makes it look like there will be paths that were matched before that won't be now.  Can this be made to support both?

Also, for regexes like this, I'm a bit fan of using rubular to create a permalink with an example that I embed in a comment.  Example:

```ruby
if line !~ %r{^\S+/gems/\S+:\d+: warning:} # http://rubular.com/r/cWWgOu3WTJ
  # ...
end
```

Makes it easy to see what cases you were thinking of when you wrote the regex for later when you come back to it.