Play Framework 2.6 support

[pwyczes]

Hi there.
In our company, we are currently in a process of migrating our Play modules from 2.3 to 2.6 (yup, huge jump).

We managed to upgrade almost everything, except play2war of course. :)

This is very important for us and therefore I am willing to work on making play2war work with Play 2.6.

Is there any specific procedure to follow? :)

Also, anyone who is willing to help is welcome. :)

PS.
I am aware of #308, but I will work on 2.6 only.