Pin rack-test to the actual upstream

They've merged our changes in upstream so we can drop our fork.
Add Google's diff patch match js library

So that we can do inline diffs without the new lines
remove diffy and use google's js diff library

This enables editors to see changes inline more akin to MS words
'redline'. The unix diff library duplicates lines which can make it
harder to see the actual change. To achieve this we use the default
settings of dpm which are:

  Diff_EditCost = 4;
  Match_Threshold = 0.5;

And the diff_cleanupEfficiency algorithm.

The API docs are here:
https://code.google.com/p/google-diff-match-patch/