PEXBuilder does not respect interpreter when compiling source to bytecode

[jsirois]

Today when calling add_source on a PEXBuilder, any '.py' files are byte code compiled using the current ambient interpreter and not the PythonInterpreter the PEXBuilder was constructed with: https://github.com/pantsbuild/pex/blob/v1.0.0/pex/pex_builder.py#L136

This leads to problems when the PEXBuilder is being used in an interpeter that has language incompatibilites with the target interpreter; ie: when running under 2.7 and trying to contruct a pex targeted to 3.4 using python 3 only language features. An example of this is outlined in pantsbuild/pants#1658

[jsirois]

Fix submitted @ ac3c7f1