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.