According to http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7078419
JDK 7 and up will never support classfile formats 48 or earlier because
they lack generic signature attributes. Our current baseline is JDK 5
anyway which has the classfile format 49
(see e.g. http://en.wikipedia.org/wiki/Java_class_file or the JVM spec)

Suggested by: Pedro Giffuni <pfg@apache.org>