Extend tox config to run without extensions.

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 downloadcache = {toxworkdir}/cache/
-envlist = py27, pypy
+envlist = py27, pypy, py27-noext, pypy-noext
 
 [testenv]
 deps =
@@ -18,3 +18,9 @@ setenv =
 [testenv:py25]
 setenv =
     PIP_INSECURE=1
+
+[testenv:py27-noext]
+commands = make check-noextensions
+
+[testenv:pypy-noext]
+commands = make check-noextensions