restrict ActiveRecord to versions 3.x

After removing Gemfile.lock, the test suite fails on Ruby 2, because
activerecord 2.3 fails on Ruby 2.

Additionally, we cannot allow ActiveRecord 4.x yet because the
activerecord-nulldb-adapter gem does not support ActiveSupport 4.x in
any released version. See https://github.com/nulldb/nulldb/issues/47

In light of these constraints, restrict our ActiveRecord requirement to
versions 3.x order to ensure compatibility with Ruby 2 and
activerecord-nulldb-adapter.