[SPARK-2987] - Adjust build system and tests to work with scala 2.11

This patch assumes that we are on akka 2.3.5. So it does not deal with that. 

For building with scala 2.10, there is no change to build procedure. -Pscala-2.10 is enabled by default. However it might be good to enable it.

For building with scala 2.11
Run the script `dev/change-version-to-2.11.sh`. This changes all the pom's artifactID and then run 
`sbt -Pscala-2.11` 

For switching back to 2.10. run `dev/change-version-to-2.10.sh` and then follow the build steps as usual.

Everything should work as is in scala 2.11 except two modules (which are disabled only for scala 2.11), external - zeromq and kafka.
These lines change the ending _2.10 at locations like [this](https://github.com/apache/spark/blob/master/core/pom.xml#L29) to _2.11.