Specify Java 1.6 as target VM during compilation

This removes the need to have a separate Java 6 SDK install.

Previously, if `` `which javac` `` pointed to a Java 7+ javac, and you hadn't manually updated your `.drozer_config` file to point to a javac 1.6 install, dx would throw the following error when packaging an APK.

    dz> run exploit.jdwp.check
    /usr/bin/javac -cp /Users/joe/Tools/drozer/drozer/src/drozer/lib/android.jar JdwpBroker.java
    /Users/joe/Tools/drozer/drozer/src/drozer/lib/dx --dex --output 237679523de7d4aa437a3b275110db94.apk     JdwpBroker.class

    trouble processing:
    bad class file magic (cafebabe) or version (0033.0000)
    ...while parsing JdwpBroker.class
    ...while processing JdwpBroker.class
    1 warning
    no classfiles specified 
    Error whilst building APK bundle.

By passing `-source 1.6 -target 1.6` we can tell `javac` to build a Java 6 compatible class file.

An alternative fix may be to update the bundled `dx.jar`, as `dx` now support Java 7 - but I think the same error would be thrown if a user had the Java 8 SDK installed...