Upgrade pex to twitter.common.python==0.3.1
Upgrade pex to twitter.common.python==0.3.1

Testing Done:
Tested upgrade on clean and dirty master.

Reviewed at https://reviews.apache.org/r/18481/

mschenck/aurora,medallia/aurora,mschenck/aurora,shahankhatch/aurora,medallia/aurora,protochron/aurora,shahankhatch/aurora,kidaa/aurora,wfarner/aurora,crashlytics/aurora,wfarner/aurora,shahankhatch/aurora,thinker0/aurora,shahankhatch/aurora,medallia/aurora,thinker0/aurora,thinker0/aurora,wfarner/aurora,medallia/aurora,rdelval/aurora,thinker0/aurora,rosmo/aurora,mkhutornenko/incubator-aurora,mschenck/aurora,crashlytics/aurora,mkhutornenko/incubator-aurora,rosmo/aurora,thinker0/aurora,kidaa/aurora,rdelval/aurora,apache/aurora,rosmo/aurora,apache/aurora,medallia/aurora,crashlytics/aurora,rdelval/aurora,apache/aurora,apache/aurora,protochron/aurora,protochron/aurora,apache/aurora,crashlytics/aurora,apache/aurora,rdelval/aurora,mkhutornenko/incubator-aurora,kidaa/aurora,mschenck/aurora,mkhutornenko/incubator-aurora,mkhutornenko/incubator-aurora,kidaa/aurora,rosmo/aurora,crashlytics/aurora,protochron/aurora,kidaa/aurora,rosmo/aurora,medallia/aurora,wfarner/aurora,wfarner/aurora,kidaa/aurora,shahankhatch/aurora,shahankhatch/aurora,wfarner/aurora,mschenck/aurora,rdelval/aurora,protochron/aurora,rdelval/aurora,thinker0/aurora,rosmo/aurora,protochron/aurora,crashlytics/aurora,mschenck/aurora