scalac warnings

[yanns]

[info] compiling 10 Scala sources to /Users/yannsimon/projects/sangria-graphql-org/sangria-federated/target/scala-2.13/classes ...
[warn] /Users/yannsimon/projects/sangria-graphql-org/sangria-federated/src/main/scala/sangria/federation/_Any.scala:21:15: abstract type Node in type pattern sangria.federation.NodeObject[Node] is unchecked since it is eliminated by erasure
[warn]       case n: NodeObject[Node] =>
[warn]               ^
[warn] /Users/yannsimon/projects/sangria-graphql-org/sangria-federated/src/main/scala/sangria/federation/Federation.scala:68:44: type Traversable in package scala is deprecated (since 2.13.0): Use Iterable instead of Traversable
[warn]       override def getMapKeys(node: Node): Traversable[String] = default.getMapKeys(node)
[warn]                                            ^
[warn] two warnings found

We should make scalac warnings fatal errors when CI is running.