Revert 'prepare' to 'prepublish'

Travis uses a so ancient version of npm that it doesn't support
'prepare', so we have to continue using 'prepublish' for now.

package.json

@@ -21,7 +21,7 @@
   "scripts": {
     "lint": "eslint app core po tests utils",
     "test": "karma start karma.conf.js",
-    "prepare": "node ./utils/use_require.js --as commonjs --clean"
+    "prepublish": "node ./utils/use_require.js --as commonjs --clean"
   },
   "repository": {
     "type": "git",