Allow environments other than prod for mix release

[sitch]

Currently the --env=prod setting is hard coded into the upgrade task, would be useful for staging/etc environments to allow "--env=#{env}"

[sitch]

For anyone else having issues using a staging or other env, you can just soft link the prod directory, i.e. for a project called "my_app", and env "staging":
ln -s ~/my_app/_build/staging ~/my_app/_build/prod