Simplified tools jar resolution.

On OSX the Oracle JDK 6 ships with ../Classes/classes.jar but Oracle JDK 7 ships with ../lib/tools.jar. Until maven supports profile activation as the docs claim this seem to work.
Tested it with:
 - compiles ok & tests pass on OS X 10.7 with Oracle JDK 7
 - compiles ok but Java 7 specific tests fail on OS X 10.7 with JDK 6
 - compiles ok & tests pass on Ubuntu 12 with Open JDK 7 & Oracle JDK 7 (https://travis-ci.org/decbis/cobertura/builds/11000007)
 - compiles ok but Java 7 specific tests fail on Ubuntu 12.04 with OpenJDK 6 (https://travis-ci.org/decbis/cobertura/jobs/10999071)