Attempt at resolving GraphQL dependency issue

To address issues #3 and #4 per @benjie's comment in #4

Author: ericmand

package.json

@@ -35,10 +35,12 @@
   "dependencies": {
     "apollo-link": "^1.2.3",
     "gatsby-source-graphql": "^2.0.5",
-    "graphql": "0.13.x",
     "pg": "^7.6.0",
     "postgraphile": "^4.1.0-rc.0"
   },
+  "peerDependencies": {
+    "graphql": "^0.13.x"
+  },
   "devDependencies": {
     "babel-eslint": "^10.0.1",
     "eslint": "^5.8.0",