Updates to be compatible with Rails 4.

* Updates the activerecord dependency
* Changes ActiveRecord::Base::ConnectionSpecification to ActiveRecord::ConnectionAdapters::ConnectionSpecification. This was changed in rails commit dae7b6545372cba40e08554b9a7b2f391eaa5c6e

I don't know how much you're interested in rails 4 compatibility, but it seems to be pretty straightforward-- this was all I had to do to get the tests passing.

Thank you for this gem!