Use JDK tools.jar matching source version (1.7 for AS8).

com.sun:tools.jar is hardcoded to be 1.6, which leads to "boot classpath needs to match -source 1.6" warnings. With using the maven.compiler.source version (which is also used by the java version enforcer) the tools.jar should match the rest of the compile world.

Discussion: https://community.jboss.org/message/807105#807105