Update sbt-assembly to 2.3.1 (#152)

Author: scala-steward

project/plugins.sbt

@@ -8,7 +8,7 @@ addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.3.1")
 
 addSbtPlugin("com.timushev.sbt" % "sbt-updates" % "0.6.4")
 
-addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.3.0")
+addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.3.1")
 
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.1")