Drop support for 1.8, 1.9, and 1.10

[mblayman]

DJA 2.3.0 supports 1.8 - 1.11. At the end of the month, Django 1.10 will no longer be supported by the Django core team and the 1.8 LTS support is ending early next year. Since the 2.3.0 release can act as a transition release, I think it's time for future versions of DJA to support 1.11 and greater. This will permit the project to clean out some weird import hoop jumping that was required to support all these versions.