Migrate to Xtext 2.6

Some things we might want to look into:

- I know there are some classes in Jnario which were copy-pasted from Xtend. I have not updated those. You might want to check whether there is interesting new stuff in Xtend that you want to copy. 
- Anonymous classes compile to broken Java code when used in Specs and Features. We could either disable them for Jnario or try to fix them.
- Jnario's update site needs to contain all necessary libraries. That probably means mirroring ASM from Orbit
- I don't know why this was done, but the fact that the Jnario Maven plugin also generates Xtend constantly bothers me. It is also a source of a lot of code duplication. What's the use case for that? Calling Jnario-generated classes from Xtend doesn't sound very common.