Modify cobertura maven plugin definition to use version 2.5

Upgrade jboss-parent to use version 2.5 for cobertura maven plugin.
Version 2.5 contains functionality for agregated report - http://jira.codehaus.org/browse/MCOBERTURA-65

Don't use version 2.5.1 because there is NPE bug - see http://jira.codehaus.org/browse/MCOBERTURA-151

Workaround: specify cobertura plugin version from command line
mvn -Pcode-coverage -Dcobertura.aggregate=true -Dversion.cobertura.plugin=2.5 clean cobertura:clean cobertura:cobertura