updating compass-rails dependency to v1.1.2 w/ rails4 support

compass-rails v2.0.alpha.0 has been yanked from rubygems.org in favor of
v1.1.2 which has been released w/ rails 4 support.

updating version dependency in gemspec and corresponding documentation

see:

* https://github.com/Compass/compass-rails/pull/113
* https://github.com/Compass/compass-rails/issues/116