chore(package): update pretty-format to version 21.0.0

Closes #23

Author: greenkeeper[bot]

package.json

@@ -56,7 +56,7 @@
     "jest": "^19.0.2",
     "jest-diff": "^19.0.0",
     "jest-matcher-utils": "^19.0.0",
-    "pretty-format": "^19.0.0"
+    "pretty-format": "^21.0.0"
   },
   "jest": {
     "collectCoverageFrom": [