Extract repo name from URL, not from DOM

The repository name is currently being extracted from an HTMLElement
that should be present in the DOM (but that currently is not present in
every page). Change this method for one that uses the URL to extract
the repository name, so it's more trustworthy.

src/github-sublime.js

@@ -18,7 +18,11 @@
    * Repository Name (we get this from the DOM)
    * @type {string}
    */
-  var repoName = $('.js-current-repository').text();
+  var repoName;
+  var pathnameParts =  window.location.pathname.split('/');
+  if(pathnameParts.length > 2) {
+    repoName = pathnameParts[2];
+  }
 
   // we can't continue if we could get a repository name
   if(!repoName) {