Release 1.2.8

[kwlzn]

docket:

• Minimum setuptools version should be 20.3 (Minimum setuptools version should be 20.3 #391)
• Improve wheel support in pex. (Improve wheel support in pex. #388)

[kwlzn]

this is complete: https://pypi.python.org/pypi/pex/1.2.8