Fix OSX build.

Without this the build on OSX with Java 6 fails as follows:
[ERROR] Failed to execute goal on project hawtio-local-jvm-mbean: Could not resolve dependencies for project io.hawt:hawtio-local-jvm-mbean:bundle:1.2-SNAPSHOT: Could not find artifact com.sun:tools:jar:1.6 at specified path /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/../lib/tools.jar