Replace 'SubfieldBase' with 'from_db_value()'

[shanecav84]

SubfieldBase is deprecated as of Django 1.8 and will be removed in Django 1.10. This pull request simply removes the EncryptedFieldMixin metaclass variable and its SubfieldBase value, and adds _from_dbvalue().

Since we don't need to do anything with our field values beyond decrypting them, _from_dbvalue() simply calls _topython() to decrypt the data and return the proper Python object.

Other, minor updates:

• The Travis CI config includes Django 1.8 and 1.9
• python-keyczar is now set to 0.715 in requirements.txt, the latest version as of this commit
• Line breaks for PEP8 compliance

[defrex]

This is excellent, thank you.

[shanecav84]

Awesome. Thanks! 🎉