[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 wee need to
build module out of the box without modification of ~/.m2/settings.xml

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

Downloading:
http://repository.jboss.org/nexus/content/groups/public/org/jboss/tools/parent/4.2.0.Alpha1-SNAPSHOT/maven-metadata.xml 
Downloaded:
http://repository.jboss.org/nexus/content/groups/public/org/jboss/tools/parent/4.2.0.Alpha1-SNAPSHOT/maven-metadata.xml
(612 B at 2.1 KB/sec) Downloading:
http://repository.jboss.org/nexus/content/groups/public/org/jboss/tools/parent/4.2.0.Alpha1-SNAPSHOT/parent-4.2.0.Alpha1-20130718.151740-7.pom 
Downloaded:
http://repository.jboss.org/nexus/content/groups/public/org/jboss/tools/parent/4.2.0.Alpha1-SNAPSHOT/parent-4.2.0.Alpha1-20130718.151740-7.pom
(32 KB at 101.3 KB/sec)