fix: replace .size() with .length

As per: magoosh#27

Author: davegudge

app/assets/javascripts/jquery.infinite-pages.js.coffee

@@ -62,7 +62,7 @@ Released under the MIT License
     # load event if close enough
     check: ->
       nav = @$container.find(@options.navSelector)
-      if nav.size() == 0
+      if (if typeof nav.size == 'function' then nav.size() else nav.length) == 0
         @_log "No more pages to load"
       else
         windowBottom = @$context.scrollTop() + @$context.height()