Add bundler 1.15.4, remove bundler 1.15.3
Add bundler 1.15.4, remove bundler 1.15.3

source url: https://rubygems.org/downloads/bundler-1.15.4.gem
source sha256: fad17ea3a1c15df2f2a7fcea052b35ebeab0dc87906cd762470637bef8c98472

[#150383252]

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