Merge pull request #677 from scala-steward/update/sbt-scalafmt-2.6.0

sbt-scalafmt 2.6.0 (was 2.5.6)

Author: mkurz

project/plugins.sbt

@@ -2,4 +2,4 @@ addSbtPlugin("org.playframework" % "sbt-plugin"   % "3.0.10")
 addSbtPlugin("com.github.sbt"    % "sbt-digest"   % "2.1.0")
 addSbtPlugin("com.github.sbt"    % "sbt-gzip"     % "2.0.0")
 addSbtPlugin("com.github.sbt"    % "sbt-stylus"   % "2.0.1")
-addSbtPlugin("org.scalameta"     % "sbt-scalafmt" % "2.5.6")
+addSbtPlugin("org.scalameta"     % "sbt-scalafmt" % "2.6.0")