Comment out Gittip link on gem pages

We don't yet have the landing pages on the Gittip side needed to support
the level of indirection to show Gittip accounts associated with a Ruby
gem:

    https://github.com/gittip/www.gittip.com/issues/1591

This comments out the link from gem pages for now. The link from
profile pages on Rubygems to Gittip is fine.