Tox: add check-manifest environment

tox.ini

 [tox]
-envlist = py27,py3,flake8
+envlist = py27,py3,flake8,check-manifest
 
 [testenv]
 deps =
@@ -7,6 +7,13 @@
 commands =
   {envpython} -m pytest {posargs}
 
+[testenv:check-manifest]
+skip_install = true
+deps =
+  check-manifest
+commands =
+  {envpython} -m check_manifest
+
 [pytest]
 python_files = *test_*.py
 testpaths = test