java: Changed JNI version to 1.6

This change is needed to compile/run with OpenJDK 7, as well as with
OpenJDK 8.

Signed-off-by: Andrei Vasiliu <andrei.vasiliu@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/java/mraajava.i

@@ -71,7 +71,7 @@ class Spi;
         /* initialize mraa */
         globVM = vm;
         mraa_init();
-        return JNI_VERSION_1_8;
+        return JNI_VERSION_1_6;
     }
 %}