Upgrading Netty

[Sanne]

Upgrading Netty and some maven plugins.

I'd like to get this working on Java 9; there is still much more to do but this is a start.

[coveralls]

Coverage decreased (-0.2%) to 59.79% when pulling 0082d45 on Sanne:UpgradeNetty into a9c3f7b on asciidoctor:master.

[abelsromero]

Thanks a lot, that's been a pending task for a long time.
Just to double check, I also run the tests in a Java6 HotSpot machine and everything works fine 👍

The current plan is to drop support for Java6 with the 1.6.0 version.

[mojavelinux]

👍

[Sanne]

thanks!