Require Python distribute >= 0.6.10

This is the version that is present on SL6. If the distribute < 0.6.10
is installed, or if distribute is completely absent, then distribute
0.6.10 will be downloaded when setup.py is run. If distribute >= 0.6.10
is present, then nothing is downloaded. This is important because when
building RPMs we don't want to require a network connection.