Upgrade pants and commons, fix python thrift namespace so that it does not conflict.

Pins twitter.common on 0.3.0 (save twitter.common.python which is released
out of band.)

Upgrades aurora to twitter.pants 0.0.21 which uses
twitter.common.python==0.5.4 and produces wheels.

Updates the thrift python namespaces so that the autogenerated python
artifacts do not clobber each other.

Reviewed at https://reviews.apache.org/r/20583/