Downgrade pex to 1.1.2

There's a bug in pex versions 1.1.3 and 1.1.4 that prevents eggs with '_'s in the version metadata in their file name from being discovered.

See pex-tool/pex#226 for more info.

Testing Done:
Reverted the version upgrade and saw the pex creation work again for a requirement with a _

Bugs closed: 3184

Reviewed at https://rbcommons.com/s/twitter/r/3687/

3rdparty/python/requirements.txt

@@ -10,7 +10,7 @@ mock==1.3.0
 mox==0.5.3
 pathspec==0.3.4
 pep8==1.6.2
-pex==1.1.4
+pex==1.1.2
 psutil==3.1.1
 pyflakes==1.1.0
 Pygments==1.4