fix(package): update joi to version 13.7.0

Closes #266

Author: greenkeeper[bot]

package.json

@@ -16,7 +16,7 @@
     "filewrap": "^1.0.0",
     "glob": "^7.1.2",
     "hygienist-middleware": "^0.1.3",
-    "joi": "^12.0.0",
+    "joi": "^13.7.0",
     "lodash.difference": "^4.5.0",
     "lodash.merge": "^4.6.0",
     "lodash.union": "^4.6.0",