Move urijs from devDependencies to dependencies

thelounge#1276 refactored previews, and in particular [reused our link-in-messages detection](https://github.com/thelounge/lounge/pull/1276/files#diff-4805d608cbc31851a7bee1bf4c7e247eR6) instead of [having a separate logic](https://github.com/thelounge/lounge/pull/1276/files#diff-4805d608cbc31851a7bee1bf4c7e247eL19). 

Unfortunately, this loads a client library, which requires `urijs`. Since these are built at release time, we were not including this package in the production dependencies, and it is now breaking at install time.

This might happen again if we add a client dependency in this file and forget it is also loaded by the server. We could in the future either extract this logic into a shared location, or we could move this logic entirely on the server (or maybe many other options), but in the meantime this will fix the issue in `v2.4.0-rc.1`.

package.json

@@ -59,7 +59,8 @@
     "request": "2.81.0",
     "semver": "5.4.1",
     "socket.io": "1.7.4",
-    "spdy": "3.4.7"
+    "spdy": "3.4.7",
+    "urijs": "1.18.10"
   },
   "devDependencies": {
     "babel-core": "6.25.0",
@@ -81,7 +82,6 @@
     "socket.io-client": "1.7.4",
     "stylelint": "8.0.0",
     "stylelint-config-standard": "17.0.0",
-    "urijs": "1.18.10",
     "webpack": "3.4.1"
   }
 }