Adapt Spoofax 2 to changes in Stratego incremental compiler

[Gohla]

Adds a dependency to stratego.build.spoofax2, and changes to work with newest version of PIE.

Needs to be merged with:

• https://github.com/metaborg/stratego/pull/10
• https://github.com/metaborg/spoofax-eclipse/pull/18
• https://github.com/metaborg/spoofax-deploy/pull/29