Minimal amount of work to make tests run with AR 4

Right now, there are a lot of deprecation warnings and a couple of
failing specs.

Run with:
`BUNDLE_GEMFILE=gemfiles/active_record_40.gemfile rake test`

This commit also adds an extra development dependency for the
`rails-observer` gem, so that we run the exact same specs than the 3.0
branch.
Make tests pass on AR 4.