Fail gracefully when release tests run without RELEASE_TESTING set

Addresses stuff like
http://www.cpantesters.org/cpan/report/57b18c6a-f748-11e1-beae-ecb8745b8025
http://www.cpantesters.org/cpan/report/2a0a3df4-f642-11e1-bb01-e3918809bd53