Pluggable caches and Python 3.5 support

• Dropped support for Django prior to 1.6
• Dropped support for Celery prior to 3.1
• Dropped support for Python 2.6
• Dropped support for Python < 3.5 - thanks @midnightlynx for 3.4/3.5 support
• Support for pluggable caches - thanks @rhunwicks
• Fixed setup.py license to match MIT license in repo - thanks @sodre