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Thanks to Chad Dombrova for pointing this out

library_builders.sh script in fact needs common_utils.sh.  Source that
script, and put a guard in common_utils.sh to prevent it being sourced
more than once.

Wrong indentation causing error in variable definition - see
https://github.com/matthew-brett/manylinux-builds/pull/10#issuecomment-233305951

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

The thing that sets the Python version for building and testing is the
MB_PYTHON_VERSION variable.

Build / test Python versions conceptual separable in case of Linux
builds.

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.