Use Spring IO platform for dependency management

See http://platform.spring.io/platform/ for more information about it.

This patch makes Spring.IO usable with ARSnova Backend and also updates
some dependiencies not beeing upgraded in ARSnova Backend POM
so I removed explicit version declaration.

Problems with json-path-assert dit not occure, so i removed version
declaration and use managed version 2.0.0.

Remove obsolete version properties

Closes GH-17. This is a squshed version of PCV's PR. (DG)