synergy: Update to new upstream version 1.4.15.

This is a bugfix release which fixes the following bugs:

 * Synergy Service - Error 87: The parameter is incorrect.
 * Option not supported on Linux: --enable-drag-drop (server not
   starting)

Bug IDs and the announcement can be found at:

http://synergy-foss.org/blog/synergy-1-4-15-released/



Signed-off-by: aszlig <aszlig@redmoonstudios.org>