Add gwt-maven-plugin in community bom

Manage the gwt-maven-plugin version to the same version as gwt artifacts which already defines in community bom.

I have seen the following error during product build:

[INFO] --- gwt-maven-plugin:2.5.1:compile (gwt-compile) @ uberfire-webapp ---
[WARNING] You're project declares dependency on gwt-user 2.5.0. This plugin is designed for at least gwt version 2.5.1
[WARNING] You're project declares dependency on gwt-user 2.5.0. This plugin is designed for at least gwt version 2.5.1
..