Remove controller actions module, and active admin from the gemfile.

A complete controller would be more useful/idiomatic, and may appear later. For
now, activeadmin is holding us back from running the test suite against Rails
4.

Also pin gems so the test suite runs after a fresh bundle.
RSpec 3.
Use in-memory database, load test schema instead of migrating.
Move test models out of dummy app.
Trim down the dummy app.
Make the test suite runnable against Rails 4.
Fix spec failures under Rails 4.0.
Fix spec failures under Rails 4.1.
Silence RSpec 3 deprecation warnings.
Travis configuration to test against multiple Rails versions.
Just test MRI.

The sqlite3 doesn't exist on JRuby, and Rubinius is still pretty fringe.
More explicit timestamps in spec.

This was failing on Travis. Perhaps due to a sqlite3 version that handles
millisecond precision differently to my local version.
Try comparing timestamps without milliseconds for Travis.
Merge pull request #37 from howaboutwe/rails4

Rails 4 compatibility, remove controller actions, in-memory test db, fix gemfile, Travis config