Add support for Travis
Merge pull request #6 from jonallured/travis

Add support for Travis
Updates changelog.
Removes dependency on ActiveModel.

We don't need to depend on `ActiveModel` to validate the two most
important keys are set at initialization.

Now, resorting to raising errors if these keys are missing.
Adds condition check on Ruby version.
Fixes Travis badge.
Adds HoundCI yml file.
Factor out some duplication