[2.9.x] Scala Steward: Pin dependencies

[mkurz]

To avoid PRs like:

• [2.9.x] play, play-ahc-ws, ... 3.0.3 (was 2.9.3) #317
• [2.9.x] play-functional, play-json 3.0.3 (was 2.10.5) #318