fix(deps): update dependency deepmerge to v4

Author: renovate-bot

package.json

@@ -33,7 +33,7 @@
   "homepage": "https://github.com/thebigredgeek/apollo-resolvers#readme",
   "dependencies": {
     "assert": "^2.0.0",
-    "deepmerge": "^3.0.0"
+    "deepmerge": "^4.0.0"
   },
   "devDependencies": {
     "babel-cli": "6.26.0",