force use of django<1.5 and added STATSD_PREFIX support for moz_metlog

    * added support to moz_metlog client to support the STATSD_PREFIX in django settings
    * forced the use of django < 1.5 for travis
    * require STATSD_PREFIX to be defined with the moz_metlog client
    * removed check for self._prefix from moz_metlog
    * cleanups in the testcases to use Django TestCase to clobber settings
    * bumped revision to 0.3.8.6