Update sbt-coveralls to 1.3.15 (#380)

Author: scala-steward

project/plugins.sbt

@@ -2,7 +2,7 @@
 addSbtPlugin("org.scoverage" %% "sbt-scoverage" % "2.3.0")
 
 // https://github.com/scoverage/sbt-coveralls
-addSbtPlugin("org.scoverage" %% "sbt-coveralls" % "1.3.14")
+addSbtPlugin("org.scoverage" %% "sbt-coveralls" % "1.3.15")
 
 // https://github.com/sbt/sbt-release
 addSbtPlugin("com.github.sbt" % "sbt-release" % "1.4.0")