railties is a better indicator of the rails gem

people are building rails apps that don't include the rails
meta gem. railties is not used outside of rails and can be
used to indicate the version of rails being used.
check that the Gemfile + Gemfile.lock exist before trying to read those files
rails 2 doesn't have railties
[#45453049] adds a PackageFetcher module to Ruby buildpack
Merge remote-tracking branch 'heroku/master'
bump to v53
update changelog
[#45453049] use PackageFetcher for all needed packages
Merge remote-tracking branch 'heroku/master'