GitLab

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.