bump bytefmt
bump bytefmt

[finishes #160270164](https://www.pivotaltracker.com/story/show/160270164)

Submodule src/code.cloudfoundry.org/bytefmt b31f603f..2aa6f33b:
  > Performance optimizations (without regexp / fmt)

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