[JBIDE-15280] Update parent/pom.xml version to 4.2.0.Alpha1

updated parent pom version and added JBoss Public Group repo
http://repository.jboss.org/nexus/content/groups/public which according to
https://community.jboss.org/wiki/MavenRepository has everything we need to
build module out of the box without modification of ~/.m2/settings.xml

Build seems to be running with brand new local repo configured with
-Dmaven.repo.local=path/to/the/new/repository