buildpacks/ruby: Fix rvm archives cache symlinking
buildpacks/ruby: Remove --binary flag to allow any ruby to be installed
buildpacks/ruby: Use system libraries when installing nokogiri
buildpacks/ruby: Make sure an stable rvm version gets installed
buildpacks/ruby: Cache gems tarballs
contrib/squid3: Use $HOME/devstep-cache/squid3 as root because keeping it on /tmp is giving me lots of trouble
Update CHANGELOG
docs: Update Ruby buildpack docs