chore: incrementing version number - v1.15.4

(cherry picked from commit e0f77d1791931073adfee2b0570fc009d0a2a580)
Signed-off-by: Misty (Bot) <deploy@nodebb.org>

install/package.json

@@ -2,7 +2,7 @@
     "name": "nodebb",
     "license": "GPL-3.0",
     "description": "NodeBB Forum",
-    "version": "1.15.3",
+    "version": "1.15.4",
     "homepage": "http://www.nodebb.org",
     "repository": {
         "type": "git",