update rdflib-zodb to rdflib4

Ok, that is a slightly bigger commit than I originally intended.

Tests seem to run fine, even those for PyPy. (including ZODB tests).

The Python 3 tests run without ZODB, as ZODB is not yet ready for Python3, however, there are other storage backends for ZODB that might be suitable for Py3 (e.g. ZEO).

For some reason the py 3.2 fails in travis-ci with a syntax error. (not sure why it happens)