Update rspec to expect syntax, plus == to eq and improve rails version test

...in integration/stylesheets_spec.rb plus split assets in spec/unit/asset_registration_space.rb (prior change).
As you may well know, using 'expect' over 'should' is a recommended practice because of dealing with delegate/proxy objects as detailed in http://myronmars.to/n/dev-blog/2012/06/rspecs-new-expectation-syntax.
As detailed in the blog post "In the future, we plan to change the defaults so that only expect is available unless you explicitly enable should. We may do this as soon as RSpec 3.0, but we want to give users plenty of time to get acquianted with it."
I'm seeing many other newer and updated gems adopt it and I thought you might like to consider it for active admin.
There wasn't an easy replacement for the "its" one liners so I have left many of them.
Sorry for the relatively large branch.
All tests pass.  Feel free to change of course.