Improvement - automatically version upgrade.

Store versions in properties allow us to use versions plugin to perform the update automatically.

After this change we can simply run:

    mvn versions:display-plugin-updates 
- to see which plugins have newer versions

and the finally:

    mvn versions:update-properties 
- to upgrade all plugins by one command

There is possibility to manage how versions are resolved by ruleset file.

If my idea is ok for you I can prepare change for the other plugins to store theirs versions in properties.