fix(gatsby-link): correct dependency declaration (#3707)

* fix(gatsby-link): correct dependency declaration

The code depends on `react-router-dom`, and therefore it should be declared as a
direct dependency, not a dev one. Also, it needs `@types/react-router-dom` for
index.d.ts.

* chore(gatsby-link): declare gatsby v1 as a peer dependency

packages/gatsby-link/package.json

@@ -17,10 +17,13 @@
   "license": "MIT",
   "devDependencies": {
     "babel-cli": "^6.26.0",
-    "cross-env": "^5.0.5",
-    "react-router-dom": "^4.2.2"
+    "cross-env": "^5.0.5"
+  },
+  "peerDependencies": {
+    "gatsby": "^1.0.0"
   },
   "dependencies": {
+    "@types/react-router-dom": "^4.2.2",
     "babel-runtime": "^6.26.0",
     "prop-types": "^15.5.8",
     "ric": "^1.3.0"