show bundler where to get dguzzo-utils

inspired by useful Yehuda post:
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and
-gemfile/
Merge branch 'master' of https://github.com/dguzzo/RMagick-Blend

Conflicts:
	rmagick-blend.gemspec