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[skip ci]

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.

For getting versions from versions with suffixes.

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

Useful for scripts overriding "build_wheel" function.

Make function to download and unpack archives.

This allows caching the archives in the `archives` directory, either by
committing to the git repo, or when doing repeated builds of the same
library.

Slight generalization of wheel building for matplotlib case where
versioneer gets confused.

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.