Support older class file versions

[broneill]

The minimum supported class file version is 51, which corresponds to Java 7. This is because custom deny actions might need to rely on MethodHandle constants. Version 50 (Java 6) can be supported by simply changing the version to 51 if the class needs to be transformed.

Supporting version 49 (Java 5) is bit more tricky, since version 50 requires the StackMapTable attribute, and the class transformer depends on this attribute as well. Updating to version 51 would require building StackMapTable attributes, which is much more difficult. It's probably not worth the effort.