Do not stop compile process on ruby warnings

[sproot]

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ruby: warning: RUBY_FREE_MIN is obsolete.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ruby: warning: RUBY_HEAP_MIN_SLOTS is obsolete.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.