Add py3-django111 testing combination

Tests are also now run in parallel using detox and pytest-xdist.

requirements/dev.txt

@@ -4,4 +4,3 @@
 -r lint.txt
 
 tox-battery>=0.5
-detox>=0.11

requirements/test.txt

@@ -2,6 +2,7 @@
 pytest>=2.9
 pytest-pythonpath>=0.7.1
 pytest-cov>=2.5.1
+pytest-xdist>=1.22.0
 # latest pip version of pytest-django is more than a year old and does not support Django 2.0
 git+https://github.com/pytest-dev/pytest-django.git@94cccb956435dd7a719606744ee7608397e1eafb
 datadiff==2.0.0

requirements/tox.txt

@@ -1,4 +1,5 @@
 # requirements for building and running tox
 tox>=2.9.1
+detox>=0.11
 
 -r setup.txt