Added MoreLikeThis handler to solrconfig.xml test cores.

The MoreLikeThis handler is missing in solrconfig.xml by default leading to 2 broken tests. Here's the stacktrace:

    $ python -m unittest2 tests
    ...............E............E........
    ====================================================================
    ERROR: test__mlt (tests.client.SolrTestCase)
    --------------------------------------------------------------------
    Traceback (most recent call last):
      File "tests/client.py", line 205, in test__mlt
        resp_body = self.solr._mlt({'q': 'id:doc_1', 'mlt.fl': 'title'})
      File "pysolr.py", line 317, in _mlt
        return self._send_request('get', path)
      File "pysolr.py", line 292, in _send_request
        raise SolrError(error_message)
    SolrError: [Reason: Error 404 Not Found]

    ====================================================================
    ERROR: test_more_like_this (tests.client.SolrTestCase)
    --------------------------------------------------------------------
    Traceback (most recent call last):
      File "tests/client.py", line 320, in test_more_like_this
        results = self.solr.more_like_this('id:doc_1', 'text')
      File "pysolr.py", line 620, in more_like_this
        response = self._mlt(params)
      File "pysolr.py", line 317, in _mlt
        return self._send_request('get', path)
      File "pysolr.py", line 292, in _send_request
        raise SolrError(error_message)
    SolrError: [Reason: Error 404 Not Found]

    --------------------------------------------------------------------
    Ran 37 tests in 5.422s

    FAILED (errors=2)