Issue #39: Fixing specs in dev environment

Fixing #39 The problem is that [the tests bump the allowable transaction count by 1](https://github.com/orslumen/record-cache/blob/master/lib/record_cache/datastore/active_record_32.rb#L18) to support database cleaner running with the default transaction strategy. Obviously this makes all tests fail in dev. Updating the spec_helper to set the env to test, since typically the spec/unit helpers in rails projects will set it to test also.