Move prop-types to dependencies.

This is the recommanded way to proceed, as metioned in https://github.com/reactjs/prop-types#how-to-depend-on-this-package.

package.json

@@ -82,7 +82,6 @@
     "pegjs": "^0.10.0",
     "preact": "^7.1.0",
     "preact-compat": "^3.9.4",
-    "prop-types": "^15.5.8",
     "pug": "^2.0.0-beta6",
     "react": "^15.5.4",
     "react-addons-css-transition-group": "^15.5.2",
@@ -118,6 +117,7 @@
   "dependencies": {
     "babel-runtime": "^6.18.0",
     "fbjs": "^0.8.8",
-    "object-assign": "^4.1.0"
+    "object-assign": "^4.1.0",
+    "prop-types": "^15.5.8"
   }
 }