Under tox, install packages with --no-deps

This should achieve two things:

* Make sure that we get the Django version we explicitly declare in
  tox.ini, rather than getting Django as an indirect dependency.

* Catch dependencies we forgot to declare or pin.

tox.ini

@@ -4,6 +4,7 @@ envlist = coverage-clean, py{27,34,35}-django{19,110}, coverage-report
 [testenv]
 # usedevelop is needed to collect coverage data.
 usedevelop = True
+install_command = pip install --no-deps {opts} {packages}
 commands = coverage run -a {envbindir}/py.test example
 deps =
     py27: -r{toxinidir}/requirements/test_py2.txt