Replace MD5 with SHA256 for Node 6.11.5
Replace MD5 with SHA256 for Node 6.11.5

Signed-off-by: Sam Coward <f2d9ddb83d985bcf331a113cd487aeceb5e618e4@pivotal.io>

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