Loosened the version required for select2-rails.

This version of select2-rails relies on sprockets ~> 2.0.0 (via sass-rails) whereas rails (~>4.1.0) relies on sprockets-rails (~>2.0) which relies on sprockets (2.12.1). A later version of select2-rails removes the dependency on sass-rails, which fixes the dependency conflict.

Tightened up version range for select2-rails.

My previous change would have allowed major version changes. Version 3.5.0 of select2-rails was the first version to not include sass-rails.

Fixes #4804