tests: fix ammeter with Rails 4

Ammeter needs to load rails before loading ammeter/init. If spec_helper loads ammeter/init before rails, this leads to a NameError crash (with Rails 4).

The way to avoid this is to load "`ammeter`" instead of "`ammeter/init`".

See https://bugzilla.redhat.com/1007631 for more information.

I'm not 100% sure that this works on Rails 3, but I've tested with Rails 4 and it allows `rspec -Ilib spec` to pass.