Add tasks to guides.rubygems.org upon release

Now guides.rubygems.org will be kept up to date with changes to the
commands or specifications when RubyGems is released.

Upon release:
* ../guides.rubygems.org is cloned
* The command and specification references are updated
* The changes to these two files are committed
* The commit is pushed

If we make maintenance releases on a future branch of master we'll need
a way to disable this functionality, perhaps checking for the master
branch will be sufficient.