Add undeclared dependencies

While trying a buildout on a project I noticed that it was failing due to ``z3c.caching`` not being declared as a dependency (and as ``plone.app.caching`` was not used it was not indirectly injected into buidlout).

On a side note I saw that there's an import for ``zope.testing`` outside tests on ``purger.py``, is that supposed to be there?

If I have time I will prepare a new pull request to add travis support.