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* Add config.pre to override envvars used in `library_builders.sh`
Fixes part 2 of https://github.com/matthew-brett/multibuild/issues/116

* rename config_pre.sh to env_vars.sh as suggested

Add notes clarifying what to check out
Addresses gh-38.

#111 dropped support

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

Thanks to Moritz E. Beber for prompting me to formalize this.

The syntax is similar to how TravisCI does it.

Travis test uses `get_macpython_enviroment` for CPython and
`get_python_enviroment` for PyPy.

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.

Fixes #14.

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

[skip ci]

[skip ci]

[skip ci]

[skip ci]

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.