use RUBYGEMS_REPO everywhere
Update SSL certificates, again.

Why did openssl give me a certificate that doesn’t work to connect? :(
Also remove the intermediate certificates that aren’t actually needed to
validate a connection to the server. Finally, include tests to make sure
that Bundler can connect to each of the important Rubygems hosts using
only the vendored certificates (h/t @luislavena for an example test).