Relicense under Apache 2.0?

[gourlaysama]

https://contributors.scala-lang.org/t/moving-to-the-apache-v2-0-license/1859

Scala-xml also went through the re-licensing (scala/scala-xml#227). I guess it would make sense to be consistent here.

[SethTisue]

indeed!

[SethTisue]

marked as "good first issue" since the scala-xml PR already exists as a model for what needs to be done. no discussions/approvals/decisions needed, only follow-through on doing the changes.

[SethTisue]

fixed by @sudohalt in #184