Fix travis matrix build

[mzuehlke]

Cleanup build.sbt and fix travis.yml to support automatic build on the last 3 scala versions:

• 2.10.x
• 2.11.x
• 2.12.x

Do we want to build on all 3 versions ?

Could a committer after merging this enable travis build for this repo here: https://travis-ci.org/TheAlgorithms/Scala

Thanks

[cclauss]

Please fix the conflicts or close.