Close #216. Switch graphql-relay to peerDependency (#261)

Author: Glavin001

package.json

@@ -33,11 +33,11 @@
   },
   "homepage": "https://github.com/mickhansen/graphql-sequelize",
   "dependencies": {
-    "graphql-relay": "^0.4.2",
     "invariant": "2.2.1",
     "lodash": "^4.0.0"
   },
   "peerDependencies": {
+    "graphql-relay": "^0.4.2",
     "graphql": "^0.5.0 || ^0.6.0"
   },
   "devDependencies": {