Upgrade to sbt-assembly version 0.9.2.

This is the most recent version of sbt-assembly available for sbt
0.12.

Among other improvements, manifests now include organization and
version information (similar to the manifest data produced by the
`package` task).
Merge pull request #26 from jparise/sbt-assembly-0.9.2

Upgrade to sbt-assembly version 0.9.2.