Change install_requires to not require Django

[PamelaM]

Addresses #44
Django being in install_requires makes it hard for a project using django_coverage_plugin and pip-compile to specify their own version of Django.

• Remove "Django" from setup.py's install_requires
• Raise RuntimeError in plugin.py if Django version is lower than 1.8

[nedbat]

Seems reasonable, but kind of unfortunate. :)