[misc] explicitly install underscore

Use the same (outdated) version that is already in place.

services/real-time/package.json

@@ -34,7 +34,8 @@
     "request": "^2.88.2",
     "settings-sharelatex": "^1.1.0",
     "socket.io": "https://github.com/overleaf/socket.io/archive/0.9.19-overleaf-4.tar.gz",
-    "socket.io-client": "https://github.com/overleaf/socket.io-client/archive/0.9.17-overleaf-3.tar.gz"
+    "socket.io-client": "https://github.com/overleaf/socket.io-client/archive/0.9.17-overleaf-3.tar.gz",
+    "underscore": "1.7.0"
   },
   "devDependencies": {
     "bunyan": "~0.22.3",