Added Java 7 instrumentation manifest attributes

After upgrading to OpenJDK 7 I can't retransform classes using
    InstrumentationSavingAgent.getInstrumentation().addTransformer(t, true);

After some research I feel that spring-instrument is missing the required manifest attributes for Java 7 (see http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html)

I'm neither an expert for instrumentation nor Gradle but I thought I'd give it a shot to at least start a discussion.