Upgrade to ASM 6.0

[smarter]

Released two weeks ago, changelog: http://asm.ow2.org/history.html. "Invalid class files generated for large methods (regression introduced in 5.2)" sounds important.

[lrytz]

Note that this repo is no longer in use, we use https://github.com/scala/scala-asm now. I'll do a scala-asm 6 release soon.