Upgrade to scala-maven-plugin 3.1.5, scala-xml 1.0-RC3

This newer version of the scala plugin for maven doesn't crash on a modularized scala (scala/scala#2855).

Full disclosure: I wouldn't go as far as saying that 3.1.5 fully supports a modularized Scala -- a fix is in the works for 3.1.6

When I submitted this, scala-xml hadn't synched to maven central yet (it has now). That reminds me: does the build use the sonatype resolver? Could one be added for faster round-trips?

Review by @dotta