Update sbt-coveralls to 1.3.15

Author: aiven-scala-steward[bot]

project/plugins.sbt

@@ -12,7 +12,7 @@ addSbtPlugin("com.github.sbt"                    % "sbt-pgp"                  %
 addSbtPlugin("com.github.sbt"                    % "sbt-release"              % "1.4.0")
 addSbtPlugin("ch.epfl.scala"                     % "sbt-scalafix"             % "0.12.1")
 addSbtPlugin("org.scoverage"                     % "sbt-scoverage"            % "2.0.11")
-addSbtPlugin("org.scoverage"                     % "sbt-coveralls"            % "1.3.11")
+addSbtPlugin("org.scoverage"                     % "sbt-coveralls"            % "1.3.15")
 addSbtPlugin("net.vonbuchholtz"                  % "sbt-dependency-check"     % "5.1.0")
 addSbtPlugin("com.github.sbt"                    % "sbt-license-report"       % "1.5.0")