Fix reference of spork rc version
The 1.0rc0 version does not exist on Rubygems.org.

Previously, when trying to change from spork 0.9.2 to spork-rails,
if guard-spork is already installed, bundle install will fail.
After this commit, one can run 'bundle update spork' and the new
spork-rails gem will install correctly, with the news spork 1.0.0rcX