Don't install mysql-connector-python on TravisCI - issue #370

This is a short term solution to a known failure in the
unit tests (errors on Python 2.6, 2.7, 3.4, stalls on
Python 3.3 and PyPy) under TravisCI, presumably due to
an update to the version of mysql-connector-python.

e.g. TypeError: unhashable type: 'bytearray'