Removed 'therubyracer', 'therubyrhino', 'less' gems and updated 'listen'.

Hi

I removed 'therubyracer', 'therubyrhino' and 'less' from Gemfile requirements as they are no longer needed since we don't compile less stylesheets during build.

Also I've changed 'listen' version requirement to be >= instead of = as it was conflicting with 'awestruct' gem which requires version newer than 1.0. Basically, it might be worth to just remove 'listen' from Gemfile since 'awestruct' requires it anyway.