config: Don't require an extension for firefox >=52.

Starting with firefox 52, no extension is required for screensharing.
See https://wiki.mozilla.org/Screensharing

config.js

     // up to and including 41. On Firefox 42 and higher, we will run without the
     // extension.
     // If set to -1, an extension will be required for all versions of Firefox.
-    desktopSharingFirefoxMaxVersionExtRequired: -1,
+    desktopSharingFirefoxMaxVersionExtRequired: 51,
     // The URL to the Firefox extension for desktop sharing.
     desktopSharingFirefoxExtensionURL: null,