Gemfile: Allow not installing therubyracer
Gemfile: Move guard-test to development group

The guard documentation previously advised to put it into a seperate
group, but it no longer does that.
Gemfile: Remove execjs dependency, coffee-rails already depends on it
Merge pull request #960 from opf/feature/optional_therubyracer

Gemfile improvements: Optional therubyracer, guard in development group, no direct execjs