Use Tycho version 0.6.0

Changed Tycho version from 0.6.0-SNAPSHOT to 0.6.0 (i.e. release).
SNAPSHOT versions are transient and should only be used for testing
purposes only. Also removed now unnecessary <pluginRepositories/>
element from JGit parent pom.xml file.

Change-Id: Ie386b2dbcba43c1ccec10465978d12d6829c6150
Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>