Update cache keys to use JSON

Daniel Silverstonerequested to merge
danielsilverstone-ct/json-cache-key into master

Description

Update the cache key to use JSON as per https://mail.gnome.org/archives/buildstream-list/2019-February/msg00035.html

Changes proposed in this merge request:

  • Add ujson dependency
  • Switch cache keys to use ujson rather than pickle
  • Update the cache key tests

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