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>

include/java/mraajni.h

@@ -31,6 +31,9 @@ extern "C" {
 #include "jni.h"
 #include "mraa/types.h"
 
+// location for defining JNI version to use
+#define JNI_REQUIRED_VERSION JNI_VERSION_1_6
+
 void mraa_java_set_jvm(JavaVM* vm);
 void mraa_java_isr_callback(void* args);
 mraa_result_t mraa_java_attach_thread();