Drop support for Django < 1.11

tox.ini

@@ -3,25 +3,19 @@ envlist =
     # Test these environments first, to catch most errors early...
     lint
     django20-py36
-    django18-py27
+    django111-py27
     docs
     # ... then test all the other supported combinations:
     django20-py{35,py3}
-    django111-py{27,34,35,36,py2}
-    django110-py{27,34,35,py2}
-    django19-py{27,34,35,py2}
-    django18-py{34,35,py2}
+    django111-py{34,35,36,py2}
     # ... then prereleases (if available):
     django21-py{35,36,37}
     djangoMaster-py{36,37}
 
 [testenv]
 deps =
-    django18: django>=1.8,<1.9
-    django19: django>=1.9,<1.10
-    django110: django>=1.10,<1.11
-    django111: django>=1.11,<1.12
-    django20: django>=2.0,<2.1
+    django111: django~=1.11.0
+    django20: django~=2.0.0
     django21: django>=2.1a1
     djangoMaster: https://github.com/django/django/tarball/master
     # testing dependencies (duplicates setup.py tests_require):
@@ -91,9 +85,6 @@ python =
 
 [travis:env]
 DJANGO =
-    1.8: django18
-    1.9: django19
-    1.10: django110
     1.11: django111
     2.0: django20
     2.1: django21