This has the benefit that:
* Allows the installation of bundler as a default gem from rubygems to
include man pages.
* Removes the need to build man pages during our tests.
* Makes working with the manifest easier, because we only have source
controlled files, and not a mix of source control and generated files.
To make sure they never fall out of sync, we replace the previous
`man:build` CI task with a `man:check` task that makes sure the
generated man pages are up to date.
https://github.com/bundler/bundler/commit/23de1d0177
David Rodríguez