Call out 'mock' as explicit test dependency for 'tox'.

Should fix build failures on slightly-older setuptools installations,
e.g.: http://jenkins.pylonsproject.org/job/transaction/53/console.

tox.ini

@@ -7,6 +7,8 @@ envlist =
 [testenv]
 commands =
     python setup.py -q test -q
+deps =
+    mock
 
 [testenv:coverage]
 basepython =
@@ -14,10 +16,10 @@ basepython =
 commands =
     nosetests --with-xunit --with-xcoverage
 deps =
+    {[testenv]deps}
     nose
     coverage
     nosexcover
-    mock
 
 [testenv:docs]
 basepython =