Several build cleanups / improvements:

- Upgrade all subprojects to use newer SBT launch script from Spark; remove
  sbt JARs, since they will now be automatically downloaded.
- Use newer sbt, sbt-assembly, Scala, and ScalaTest versions.
- Remove unused dependency from streaming tests.
- Switch builds to use .scala build definitions instead of .sbt files; this
  will make it easier to introduce a test support library that can be shared
  across test projects.
- Add MLlib perf tests to Travis tests.
- Run `sbt assembly` in Travis tests.
- Add Travis badge to README.