[Bugfix] Assign unicorn rack environment same as rails env

@sosedoff

I ran into an error deploying to a staging environment where Unicorn would no longer start. I kept getting an error that my database configuration does not specify an adapter, which it does.

I found that currently ```unicorn_rack_env``` is currently set to 'development' OR 'deployment. My staging environment has a rails_env name of "staging".. so unicorn, on start, couldn't find the appropriate database configuration in my database.yml file.

I suggest altering it such that ```unicorn_rack_env``` is set to be the same as rails_env.