Revert "SONARPLUGINS-3170 Upgrade to Cobertura Maven plugin 2.6"

This reverts commit f68531a96783c46434ae74f0bf571a3a4e062dd1.

Cobertura 2.0.3 has a dependency on tools.jar from JDK, which doesn't
allow usage of cobertura-maven-plugin without configuration
out-of-the-box on some Macs. So actually we can't do an upgrade now as
it won't work for some users.