[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.
:+1:

Added the message only as an afterthought, that's why I didn't think of is_ci_build.