The Scala dependency is defined via Apache TinkerPop. JanusGraph is aligned currently with the TinkerPop 3.2.x branch, which is using
Scala 2.10.5 (and
Spark 1.6.1). Scala 2.11 (and Spark 2.0.2) is coming with
TinkerPop 3.3, but there isn't a schedule in place for that yet.
I'm not aware if anybody has attempted to build JanusGraph against TinkerPop master branch, but I imagine there would be some work involved for any breaking changes.