Replace maven-release-plugin by version-maven/scm

No more snapshot commits in main repository.
New way to release:

1. mvn version:set -DnewVersion=2.0.5 # replace with new version
2. git commit
3. mvn scm:tag

See for http://axelfontaine.com/blog/final-nail.html details.
Merge branch 'master' of https://github.com/kohlschutter/kohlschutter-parent