Move `mkdirp` and `rimraf` to dev dependencies

These dependencies are only needed by `npm run clean` and (indirectly)
`npm run build` which only work when the other dev dependencies are
installed.

package.json

@@ -43,6 +43,7 @@
     "lodash.isequal": "4.5.0",
     "lodash.omit": "4.5.0",
     "lodash.pick": "4.4.0",
+    "mkdirp": "^0.5.1",
     "minilog": "3.1.0",
     "postcss-import": "9.1.0",
     "postcss-loader": "1.3.3",
@@ -55,6 +56,7 @@
     "react-tabs": "0.8.2",
     "redux": "3.6.0",
     "redux-throttle": "0.1.1",
+    "rimraf": "^2.6.1",
     "scratch-audio": "latest",
     "scratch-blocks": "latest",
     "scratch-render": "latest",
@@ -67,9 +69,5 @@
     "webpack": "2.3.2",
     "webpack-dev-server": "^2.4.1",
     "xhr": "2.4.0"
-  },
-  "dependencies": {
-    "mkdirp": "^0.5.1",
-    "rimraf": "^2.6.1"
   }
 }