Add Backbone global to jshintrc

Underscore was previously defined, it seemed odd to not have Backbone as well.

.jshintrc

@@ -109,10 +109,10 @@
     // The parameter "predef" should remain empty for this configuration file
     // to remain as general as possible.
     "predef": [
-        // jQuery globals
-        "jQuery", "$",
-
-        // Underscore.js globals
+        // JavaScript global libraries
+        "Backbone",
+        "jQuery",
+        "$",
         "_",
 
         // RequireJS globals