Add Plone 4.0 and 4.1 under Python 2.6 to the testing matrix
Run CI on Python 2.6 for Plone 4.2 and Python 2.7 for Plone 4.3
Use latest version of Plone 4.3 branch for development
Bump up version of zc.buildout
Speed up CI by using standard Travis CI configuration
Fix allowed hosts