Update sbt-ci-release to 1.6.0 (#513)

Author: scala-steward

project/plugins.sbt

@@ -1,5 +1,5 @@
 addSbtPlugin("com.github.sbt" % "sbt-release"    % "1.4.0")
 addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.2")
-addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.5.12")
+addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.6.0")
 addSbtPlugin("org.scoverage"  % "sbt-scoverage"  % "2.1.0")
 addSbtPlugin("org.typelevel"  % "sbt-tpolecat"   % "0.5.1")