Only print doc install message when enabled

With the new post_installs hook documentation generation happens outside
of RubyGems (ignoring the hook for legacy RDoc).  Despite this, RubyGems
would print a message saying "Done installing documentation", even when
no hooks were present or documentation generation was disabled.

Now the hook has been pushed down to the RubyGems hook so it will only
show up when documentation generation is enabled.  A follow-up commit
will occur to RDoc.

Fixes #469