gatsby-remark-images: moved gatsby-plugin-sharp to dependencies (#1785)

Having it in peer dependencies is wrong, as @nickmaltsev mentioned
and I realised, as it's used in a source.

The only thing it does being in peerDependencies is only warning
users about missing dep, but can be easily dismissed.

packages/gatsby-remark-images/package.json

@@ -18,15 +18,13 @@
   },
   "dependencies": {
     "cheerio": "^1.0.0-rc.2",
+    "gatsby-plugin-sharp": "^1.6.1",
     "image-size": "^0.5.1",
     "is-relative-url": "^2.0.0",
     "lodash": "^4.17.4",
     "slash": "^1.0.0",
     "unist-util-select": "^1.5.0"
   },
-  "peerDependencies": {
-    "gatsby-plugin-sharp": "^1.6.0"
-  },
   "scripts": {
     "build": "babel src --out-dir . --ignore __tests__",
     "watch": "babel -w src --out-dir . --ignore __tests__"