Drop Plone 4.1 support in dexteritytextindexer 2.x

The new `searchable` directive is implemented using `plone.supermodel`'s `MetadataListDirective`, which is included in `plone.supermodel>=1.1.1`.

The dependencies of plone.supermodel now seem to require `collective.z3cform.datetimewidget>=1.1dev`, but Plone 4.1 pins it to `1.0.5` - therefore we can no longer support Plone 4.1 in `collective.dexteritytextindexer>=2`.

This pull-request removes Plone 4.1 tests, defines the Plone version classifiers (4.2 and 4.3), requires `plone.supermodel>=1.1.1` and fixes the name of the Plone 4.2 test buildout (which is actually testing dexterity 1, not 2).
See the commits / commit messages for more details.

Use the 1.x series of `collective.dexteritytextindexer` for Plone 4.1.

/cc @lukasgraf