Rails 4 support

* updates use of `ActiveModel::Name`
* fixes duplicate root route error when run from the root namespace
* includes the appropriate Sass helper depending on Rails version
* update asset pipeline check
* use the previous incarnation of #input_class for FormBuilder
* update to RSpec 2.13, which fixes some false-positive test failures on Rails 4

This does not fix the Cucumber feature failures related to Strong Parameter integration (visible in [this Travis CI build](https://travis-ci.org/agrobbin/active_admin/builds/8297607)).