declare supported Plone versions
A number of improvements to how we run builds on Travis

Among them:
* add Plone 4.0 and 4.1 under Python 2.6 to the testing matrix
* do not run Plone 4.3 with Python 2.6 and Plone 4.2 with Python 2.7 -- this
  reduces the number of builds but keeps basically the same coverage
* only upload coverage info to Coveralls.io on one job per build, not on all
  jobs
* upgrade zc.buildout to 1.7.1 (was 1.6.3)
* use Plone 4.3 by default (was 4.2)