Update sbt-coveralls to 1.3.15

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("com.evolution" % "sbt-artifactory-plugin" % "0.0.2")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.0.12")
-addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.11")
+addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.15")
 addSbtPlugin("com.github.sbt" % "sbt-release" % "1.4.0")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.3")
 addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.12.1")