[RFC] Add dependency rebuilding (32/64 bit).

This has been used to generate neovim/deps#4. Here's a full Travis test build: https://travis-ci.org/fwalch/bot-ci/builds/35124444. Output is in https://github.com/fwalch/deps/tree/testing.
Yes, this is for things that the user might want to abort locally, but Travis should do in all cases (e.g. https://github.com/neovim/bot-ci/pull/20/files#diff-eb23d815554bddb9ac4f5c48c7f8601fR60).