Use runtime JVM version for BlockReturnTypeCreator created classes

ClassBytecodeWriter determines the current running jvm's version and uses that as the target. BlockReturnTypeCreator had 1.7 as its hard coded version which was breaking the jdk 1.6 build. This (hopefully) fixes the openjdk 1.6 build by using the runtime jvm version.

It would be better to have a way to set the target version explicitly, then we could build for 1.5-6 from 1.7, but this commit doesn't add that because I don't know where that would go.

What do you think?