Peg sprockets to ~> 2.11.0 and sass to ~> 3.2.15 for Rails 4.x builds

If we don't specify 2.11.0 we'll end up with sprockets 2.12.0 in the
main Gemfile.lock but since sass-rails gets generated (rails new) into
the test app it'll want sprockets 2.11.0 and we'll have a conflict

If we don't specify 3.2.15 we'll end up with sass 3.3.2 in the
main Gemfile.lock but since sass-rails gets generated (rails new) into
the test app it'll want sass 3.2.0 and we'll have a conflict