Upgrade to less 1.4.2

This pull request updates less to v1.4.2. The gemspec references yet unreleased version of commonjs 0.2.7, so it should be released first. It is required because of this commit https://github.com/cowboyd/commonjs.rb/commit/80976045ff0e3c3dea3af7cb23c443dc4480c4d9