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This changes how the the Python implementation is selected.  It's now
primarily based on enviroment variables.

Uses the PYPY_VERSION enviroment variable to determine the version of PyPy to
install.  An example has been added to the matrix in the readme.

Dashes were missing from untar on MacOS.

Redirect fill_pyver errors to stderr, instead of returning them as the
result.

See: http://bugs.python.org/issue29065#msg283984

From suggestion by Ralf G.

Always load library builders (for OSX as well as Manylinux).

Make configuration more general.

Move responsibility for source futzing back to the travis.yml file,
because we are now doing only one build per job.