Upgrade pex

[objmagic]

Travis CI sometimes fails with the following error message related to Pex.

....
fastbuild/bin/third_party/pex/_pex.runfiles/__main__/third_party/pex/pex/resolver.py", line 254, in build
    os.rename(target + '~', target)
OSError: [Errno 2] No such file or directory
____(06-30 00:08:42.656) Building complete.
____(06-30 00:08:42.664) Elapsed time: 530.480s, Critical Path: 517.13s

It seems that this PR of Pex fixed the problem. I suggest we should upgrade Pex.