bump bbs
bump bbs

[finishes #161194270]

Submodule src/code.cloudfoundry.org/bbs e255e5a5..6f0fad86:
  > Change the case of enum values to be ALL_CAPS

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release