Fix compatibility with latest pytest-xdist - deprecate bad terminologies

• Some tests were failing probably from a old file formatting.
• pytest-xdist compatibility was fixed for latest major version: 2.0
  the bad terminologies (slaves and masters) were removed.
• Deprecated support of pytest-xdist <2.0, will be dropped on
  pytest-django-queries 2.0 release.
• Official compatibility for Python 2.7, 3.4 and 3.5 was dropped
  as many dependencies are no longer supporting those versions.
  Use this plugin on those Python version at your own risk.