Update sbt-scoverage to 2.3.1 (#383)

Author: scala-steward

project/plugins.sbt

@@ -1,5 +1,5 @@
 // https://github.com/scoverage/sbt-scoverage
-addSbtPlugin("org.scoverage" %% "sbt-scoverage" % "2.3.0")
+addSbtPlugin("org.scoverage" %% "sbt-scoverage" % "2.3.1")
 
 // https://github.com/scoverage/sbt-coveralls
 addSbtPlugin("org.scoverage" %% "sbt-coveralls" % "1.3.15")