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Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116

* 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

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116

* 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

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

This installs ccache from homebrew, activates it, and verfies its operation.

Functions renamed to match their intent better.

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.

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.

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

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.