migrates to paradise 2.0.0-M6

This should remove the source incompatibility with 2.11 introduced by recent migration to M4/M5, making specs2 eligible for dbuilding again: https://github.com/typesafehub/community-builds/pull/30.

Checked the 2.10.3 build for `specs2-matcher-extra` locally, and it passed. Not sure how to check the 2.11 build, because just running with `++2.11.0-RC3` didn't got me far.