Add bundler 1.15.2, remove bundler 1.15.1
Add bundler 1.15.2, remove bundler 1.15.1

source url: https://rubygems.org/downloads/bundler-1.15.2.gem
source sha256: 941ccbcde4749f5f89c42e726fe07fd0e96adcf9fe0e5ddebac3fae000e3f887

[#149029147]

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