relax the celluloid dep by one version

- berkshelf pulls in ridley which pulls in an overly aggressive
  sperm operator on celluloid:  celluloid (~> 0.14.0)
- since that is incompatible with the celluloid ~> 0.15 constraint
  that creates issues in bundling gems for test setups with
  test-kitchen.
- relaxing the ridley dep is probably the right thing to do, but
  as SV didn't pin TK at ~> 0.15 because 0.14.x was broken, backing
  it off by one should work (and the depsolver must have been using
  this anyway for everyone using TK+berkshelf together)