Fix e2e test's dependency on GWT BOM
Fix e2e test's dependency on GWT BOM

For some reason it works with 2.9.0 (dependencyManagement
inherited from parent POM?) but it no longer works with
2.10.0 as gwt-user is now just a relocating POM.

tbroyer/gwt-maven-plugin,tbroyer/gwt-maven-plugin