Do not stop compile process on ruby warnings

Currently plugin stops compilation process and throws an exception on ruby warnings

```
[00:56:48] Using gulpfile ~/code/myapp/gulpfile.js
[00:56:48] Starting 'slim'...
[00:56:48] Finished 'slim' after 8.53 ms
[00:56:49] gulp-notify: [Compile Error] Error in plugin 'gulp-slim'
Message:
Slim error in file (/Users/alex/code/myapp/frontend/javascripts/app/welcome/welcome.html.slim):
ruby: warning: RUBY_FREE_MIN is obsolete. Use RUBY_GC_HEAP_FREE_SLOTS instead.
ruby: warning: RUBY_HEAP_MIN_SLOTS is obsolete. Use RUBY_GC_HEAP_INIT_SLOTS instead.
```

I believe it should ignore warnings and only pay attention to the real errors.

Start with the gulpfile.js entry point and the gulp-slim compilation path shown in the report; reproduce the warning using the referenced Slim file. Done means Ruby warnings no longer stop compilation, while genuine Slim errors still produce the reported compile error.

Written by the indexing model from the issue text.