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.

The build task does not generate any files at `lib/bundler/generated`.

* Adds an extra artifice task to vendorize new `connection_pool`
dependency.

* Cherry-pick's needed Windows fix not yet merged into master branch of
`net-http-persistent`.

* Update bundler usages to be compatible with the new version, and fix
unit specs.

And refactor development setup.

7223: RUBY_ENGINE should always be defined after 1.8.7 r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that we have code checking that `RUBY_ENGINE` everytime it's used and I think it's unnecessary.

### What was your diagnosis of the problem?

My diagnosis was that every ruby implementation newer than 1.8 should define this.

### What is your fix for the problem, implemented in this PR?

My fix is to remove the unnecessary code.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>