relicense under Apache 2.0

[SethTisue]

just like e.g. scala/scala-xml#242 and scala/scala-parser-combinators#184

[SethTisue]

don't forget build.sbt will need:

+    // this line could be removed after https://github.com/scala/sbt-scala-module/issues/48 is fixed
+    licenses := Seq(("Apache-2.0", url("https://www.apache.org/licenses/LICENSE-2.0"))),