Move cross-env dep out of devDependencies (vuejs#105)

Since the module is used to bootstrap a production version of the site (see the npm `start` script), it shouldn't be inside devDependencies.

Author: iiegor

package.json

@@ -16,6 +16,7 @@
   },
   "dependencies": {
     "compression": "^1.6.2",
+    "cross-env": "^3.1.3",
     "es6-promise": "^4.0.5",
     "express": "^4.14.0",
     "firebase": "^3.6.2",
@@ -32,7 +33,6 @@
     "autoprefixer": "^6.5.3",
     "buble": "^0.15.1",
     "buble-loader": "^0.4.0",
-    "cross-env": "^3.1.3",
     "css-loader": "^0.26.0",
     "extract-text-webpack-plugin": "^2.0.0-beta.3",
     "file-loader": "^0.9.0",