Spark 2.4.0 (Scala 2.12.x) Release

[morazow]

Just recently Spark 2.4.0 was released and there are many improvements.

Interesting features from this release:

• Scala 2.12.x can be used (this should also be included into cross compilation)
• Barrier Execution Mode

@3cham , @jpizagno the barrier execution can help us with the sub connection issue. The idea is that Spark will wait until all task slots are ready to start. Even though it might not be backward compatible with previous Spark versions, we should keep an eye on barrier execution feature.

[morazow]

Hey all,

I am going to close this issue since we introduced already Spark version 2.4.0 in #37.

However, I am also going to create specific issue for Scala 2.12+. In near future, we should be able to compile with Scala 2.12.