Drop support for Plone 4.1

@frisi @petschki @tomgross please take a look on this so we can make CI easier.

I foresee the following scenarios that we will need to test:

* Plone 4.2, no plone.app.contenttypes
* Plone 4.3, no plone.app.contenttypes
* Plone 4.3, plone.app.contenttypes

right now Dexterity is always available on tests; but we will need to fix that also.

closes #36  
refs. #38