Add bundler 2.1.3, remove bundler 2.1.2
Add bundler 2.1.3, remove bundler 2.1.2

for stack(s) cflinuxfs3 [#170500652]

cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack