Update package.json

Change required dependency - express from "4.x" to ">=4.5"

Reference(security issue):

https://nodesecurity.io/advisories/express_no-charset-in-content-type-header

Author: PeterDaveHello

package.json

@@ -19,7 +19,7 @@
     "cookie-parser": "1.4.0",
     "ejs": "^1.0.0",
     "emojify.js": "^1.0.1",
-    "express": "4.x",
+    "express": ">=4.5",
     "express-session": "^1.11.1",
     "formidable": "^1.0.17",
     "highlight.js": "^8.4.0",