v1.3.7: Add support for mongoid 7 (#30)

- Add support for mongoid 7
- Includes version bump to 1.3.7
- Cleanup and broaden Travis version support
- Allow Mongoid 3 and 4 to fail in Travis. As you can see in this run, the same specs are already failing on master branch and not worth it to fix them at this point. https://travis-ci.org/KoanHealth/mongoid-encrypted-fields/builds/373817487