Stops enforcing users to use rails 4.0.x when using the alchemy command.

When using the alchemy command (bin/alchemy) users were forced to use rails 4.0.x.
With this change users can also use rails 4.x.x.
Merge pull request #522 from magiclabs/alchemy_binary_rails_version

Stops enforcing users to use rails 4.0.x when using the alchemy command.