Bump pex version to latest. (#4314)
Bump pex version to latest. (#4314)

This picks up a couple of bugfixes in pex 1.2.4.
15Dkatz/pants,twitter/pants,lahosken/pants,pombredanne/pants,wisechengyi/pants,fkorotkov/pants,pantsbuild/pants,jsirois/pants,UnrememberMe/pants,foursquare/pants,landism/pants,pantsbuild/pants,fkorotkov/pants,pombredanne/pants,tdyas/pants,peiyuwang/pants,peiyuwang/pants,UnrememberMe/pants,pantsbuild/pants,baroquebobcat/pants,wisechengyi/pants,benjyw/pants,wisechengyi/pants,wisechengyi/pants,twitter/pants,UnrememberMe/pants,landism/pants,benjyw/pants,fkorotkov/pants,UnrememberMe/pants,lahosken/pants,tdyas/pants,fkorotkov/pants,15Dkatz/pants,jsirois/pants,twitter/pants,foursquare/pants,pombredanne/pants,lahosken/pants,landism/pants,UnrememberMe/pants,wisechengyi/pants,landism/pants,wisechengyi/pants,baroquebobcat/pants,fkorotkov/pants,wisechengyi/pants,baroquebobcat/pants,lahosken/pants,tdyas/pants,foursquare/pants,pantsbuild/pants,lahosken/pants,lahosken/pants,peiyuwang/pants,tdyas/pants,UnrememberMe/pants,twitter/pants,fkorotkov/pants,landism/pants,UnrememberMe/pants,benjyw/pants,UnrememberMe/pants,fkorotkov/pants,jsirois/pants,benjyw/pants,tdyas/pants,peiyuwang/pants,pombredanne/pants,wisechengyi/pants,pantsbuild/pants,landism/pants,15Dkatz/pants,pombredanne/pants,tdyas/pants,foursquare/pants,baroquebobcat/pants,tdyas/pants,tdyas/pants,tdyas/pants,pombredanne/pants,peiyuwang/pants,landism/pants,15Dkatz/pants,baroquebobcat/pants,baroquebobcat/pants,15Dkatz/pants,pantsbuild/pants,twitter/pants,twitter/pants,peiyuwang/pants,benjyw/pants,twitter/pants,benjyw/pants,baroquebobcat/pants,foursquare/pants,benjyw/pants,peiyuwang/pants,pantsbuild/pants,pombredanne/pants,UnrememberMe/pants,twitter/pants,baroquebobcat/pants,twitter/pants,foursquare/pants,lahosken/pants,landism/pants,foursquare/pants,foursquare/pants,lahosken/pants,peiyuwang/pants,baroquebobcat/pants,fkorotkov/pants,15Dkatz/pants,15Dkatz/pants,wisechengyi/pants,foursquare/pants,15Dkatz/pants