Ensure OnDropFiles function matches interface

Fixes the following error printed to the console after dropping a file onto a text box: 
TypeError: invalid result from FileDrop.OnDropFiles(), a 'bool' is expected not 'NoneType'

gooey/gui/util/filedrop.py

@@ -8,3 +8,4 @@ class FileDrop(wx.FileDropTarget):
   def OnDropFiles(self, x, y, filenames):
     for name in filenames:
       self.window.WriteText(name)
+    return True