Update joi. Closes #32

Author: hueniverse

package.json

@@ -14,7 +14,7 @@
   "dependencies": {
     "@hapi/boom": "7.x.x",
     "@hapi/hoek": "8.x.x",
-    "@hapi/joi": "15.x.x"
+    "@hapi/joi": "16.x.x"
   },
   "devDependencies": {
     "@hapi/code": "6.x.x",