Allow environment to be set explicitly

This change allows the environment to be set explicitly prior to load,
for applications where `RAILS_ENV` or `RACK_ENV` are not appropriate:

    Armoire.environment = 'not_rack'
    Armoire.load!('application.yml')
Merge pull request #7 from seenmyfate/feature/allow-explicit-env

Allow environment to be set explicitly
Bump version to 1.3.0