Use /usr/libexec/java_home to locate Java 1.6 on OS X

On OS X Mavericks with Java 7, `grunt release` fails with:

    Running "find_native_java" task
    Locating your Java 6 installation...
    >> Java: java
    >> Javap: javap
    >> Javac: javac
    Fatal error: Detected Java 1.7.0 (via javac). Please use Java <= 1.6

since its trying to use the "default" Java installation, even though Java 6 is also installed. To find it, changed the `find_native_java.ts` task to use `/usr/libexec/java_home` to specifically ask for this version, now setup succeeds:

    Running "find_native_java" task
    Locating your Java 6 installation...
    >> Java: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/bin/java
    >> Javap: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/bin/javap
    >> Javac: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/bin/javac