[Fix bug 977556] Update version of python-memcached

Mozillians' requirements call for a version of python-memcached that apparently has disappeared from PyPI:

Could not find a version that satisfies the requirement python-memcached==1.45 (from -r requirements/prod.txt (line 5)) (from versions: 1.31, 1.47, 1.48, 1.51, 1.52, 1.53)