use ClassBytecodeWriter.BYTECODE_VERSION for the jvm version of Block return types

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 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.
try using doReturn from mockito

This test seems to be testing Mockito. I can't get it to fail locally, but it fails on travis. Maybe switching how we are stubbing the method will fix it.

The docs suggest it should be used this way
Merge pull request #181 from baroquebobcat/jdk_16_build

Use runtime JVM version for BlockReturnTypeCreator created classes