use Tycho 0.21

see http://wiki.eclipse.org/Tycho/Release_Notes/0.21

Change-Id: Ibdd61c4e34d8c9c63e9de8b9b155447a441b540e
Reviewed-on: http://gerrit.esdi-community.eu/3148
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>
Tested-by: Simon Templer <simon.templer@igd.fraunhofer.de>