Bump version to 1.4.0

Signed-off-by: Yukai Huang <yukaihuangtw@gmail.com>

package.json

 {
   "name": "codimd",
-  "version": "1.3.1",
+  "version": "1.4.0",
   "description": "Realtime collaborative markdown notes on all platforms.",
   "keywords": [
     "Collaborative",