Add instruction to install compass

When you start with a clean environment, `grunt build` will fail because compass is not present:
```
Running "compass:prod" (compass) task
/Users/xdutreilh/.rbenv/versions/2.0.0-p0/lib/ruby/2.0.0/rubygems/dependency.rb:296:in `to_specs': Could not find 'compass' (>= 0) among 65 total gem(s) (Gem::LoadError)
	from /Users/xdutreilh/.rbenv/versions/2.0.0-p0/lib/ruby/2.0.0/rubygems/dependency.rb:307:in `to_spec'
	from /Users/xdutreilh/.rbenv/versions/2.0.0-p0/lib/ruby/2.0.0/rubygems/core_ext/kernel_gem.rb:47:in `gem'
	from /Users/xdutreilh/.rbenv/versions/2.0.0-p0/bin/compass:22:in `<main>'
```

The pull request updates the README to make this dependency explicit (as well as Ruby and RubyGems). Not so much, but still useful.