Update Gemfile source
Remove rvmrc
Rename LICENSE.txt to LICENSE
Update LICENSE and version

Updating the license is a major version bump, since it could be a breaking
change for some organizations. Additionally, this API is now considered
"stable" by Semantic Versioning standards.
Update README to expand usage wih ENV (environment variables)

Based on the discussion from #4, update the README with Seth's information
on using magiconf with environment variables.