Change root and common settings to enable dist.

Remove dev from root, and remove dist settings from dev-build.
Specify -Xfatal-warnings for scala compiler.

Without the '-', the dist task in sbt reports an error (when on the project dev-build).
Update sbt assembly to 0.9.0

The assembly plugin version 0.9.0 has known issues, but the caching improves the dist time.

Notably, the current asset map system can find false map collisions due to out-of-date files being left in the assembly cache. Fixed by extending the clean-asset-maps task, which will also delete application folder cache entries.
Merge pull request #1110 from guardian/sbt-assembly-bump

Sbt assembly bump