Update tox.ini

- Drop tox envs for Django 1.10 or older.
- Drop tox env py27dj111 as Django 1.11 is our primary version
  and it is covered by py27.
- Ensure Django 2.0 is installed in py35dj20 and manage-py35dj20.
  The current install command potentially installs Django 2.1 or
  later if alpha version is released. It is better to ensure
  2.0.x is installed.

Change-Id: I85cbb61da9496313ae1633ec94b85702b434b122

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27dj{18,19,110},py35,releasenotes,npm
+envlist = pep8,py27,py35,py35dj20,releasenotes,npm
 minversion = 2.3.2
 skipsdist = True
 
@@ -32,32 +32,12 @@ setenv =
   {[testenv]setenv}
 commands = {[unit_tests]commands}
 
-[testenv:py27dj18]
-commands =
-  pip install -U django>=1.8,<1.9
-  {[unit_tests]commands}
-
-[testenv:py27dj19]
-commands =
-  pip install -U django>=1.9,<1.10
-  {[unit_tests]commands}
-
-[testenv:py27dj110]
-commands =
-  pip install -U django>=1.10,<1.11
-  {[unit_tests]commands}
-
-[testenv:py27dj111]
-commands =
-  pip install -U django>=1.11,<2.0
-  {[unit_tests]commands}
-
 [testenv:py35dj20]
 basepython = python3.5
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =
-  pip install -U --pre django
+  pip install django>=2.0,<2.1
   {[unit_tests]commands}
 
 [unit_tests]
@@ -157,7 +137,7 @@ commands = {envpython} {toxinidir}/manage.py {posargs}
 basepython = python3.5
 deps = {[testenv:manage]deps}
 commands =
-  pip install -U --pre django
+  pip install django>=2.0,<2.1
   {[testenv:manage]commands}
 
 [flake8]