HOTFIX: Remove generated-mima-excludes file after runing MIMA.

This has been causing some false failures on PR's that don't merge
correctly.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #971 from pwendell/mima and squashes the following commits:

1dc80aa [Patrick Wendell] HOTFIX: Remove generated-mima-excludes file after runing MIMA.
sbt 0.13.X should be using sbt-assembly 0.11.X

https://github.com/sbt/sbt-assembly/blob/master/README.md

Author: Kalpit Shah <shahkalpit84@gmail.com>

Closes #555 from kalpit/upgrade/sbtassembly and squashes the following commits:

1fa7324 [Kalpit Shah] sbt 0.13.X should be using sbt-assembly 0.11.X