Ignore empty/fake warnings - fixes #31

I have the newest version of sass and gul-ruby-sass and I get this error for a valid scss file:

```
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: 





  at ChildProcess.<anonymous> (/var/webuni.cz/node_modules/gulp-ruby-sass/index.js:98:25)
  at ChildProcess.EventEmitter.emit (events.js:98:17)
  at maybeClose (child_process.js:735:16)
  at Process.ChildProcess._handle.onexit (child_process.js:802:5)
```