Reduce Adhearsion named environments to a name

given the flexibility of loading configuration from environment variables, which may be modified between environments using proper configuration management systems, I have long been minded to remove the concept of environments entirely from Adhearsion. To my knowledge, there is no value provided by named environments that cannot be had equally as easily using proper loading of environment-specific config from environment variables; on this basis, I intend to remove them for Adhearsion 3.0.

* No more config per environment
* No more integration with RAILS_ENV
* Retain purely for the purpose of loading the correct groups from a Gemfile
* Set via AHN_ENV environment variable only

See #442