tox: add a check-manifest testenv

tox.ini

 [tox]
-envlist = py27, py3, flake8
+envlist = py27, py3, flake8, check-manifest
 
 [testenv]
 deps =
@@ -14,6 +14,13 @@ deps =
   flake8
 commands = flake8
 
+[testenv:check-manifest]
+skip_install = true
+deps =
+  check-manifest
+commands =
+  {envpython} -m check_manifest {toxinidir}
+
 [flake8]
 exclude = migration/*,setup.py,.tox/*
 max-line-length = 100