Fix error building node-sass on travis

Should finally fix the node-sass-related issues buildling on travis.

 * Used this as a blueprint travis.yml: https://github.com/sass/node-sass/blob/master/.travis.yml
 * More info: https://github.com/sass/node-sass/issues/497#issuecomment-62552565
 * And here: https://github.com/sass/node-sass/issues/490