Update sbt-ci-release to 1.5.7 (#271)

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("org.scalariform" % "sbt-scalariform" % "1.8.3")
 addSbtPlugin("com.typesafe.sbt" % "sbt-osgi" % "0.9.6")
 addSbtPlugin("com.scalapenos" % "sbt-prompt" % "1.0.2")
-addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.5.6")
+addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.5.7")
 addSbtPlugin("com.dwijnand" % "sbt-travisci" % "1.2.0")
 addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.5.3")