Update sbt-scalafmt to 2.3.1 (#115)

Author: scala-steward

project/plugins.sbt

@@ -1,4 +1,4 @@
 addSbtPlugin("com.dwijnand"      % "sbt-dynver"   % "4.0.0")
 addSbtPlugin("org.foundweekends" % "sbt-bintray"  % "0.5.6")
 addSbtPlugin("de.heikoseeberger" % "sbt-header"   % "5.4.0")
-addSbtPlugin("org.scalameta"     % "sbt-scalafmt" % "2.3.0")
+addSbtPlugin("org.scalameta"     % "sbt-scalafmt" % "2.3.1")