Add coveragePluginVersion key

Make it easier to change version of scalac-scoverage-plugin dependency

Author: lustefaniak

src/main/scala/scoverage/ScoverageKeys.scala

@@ -17,4 +17,5 @@ object ScoverageKeys {
   lazy val coverageOutputDebug = settingKey[Boolean]("turn on the debug report")
   lazy val coverageCleanSubprojectFiles = settingKey[Boolean]("removes subproject data after an aggregation")
   lazy val coverageOutputTeamCity = settingKey[Boolean]("turn on teamcity reporting")
+  lazy val coveragePluginVersion = settingKey[String]("version of scalac-scoverage-plugin to use")
 }

src/main/scala/scoverage/ScoverageSbtPlugin.scala

@@ -40,7 +40,8 @@ object ScoverageSbtPlugin extends AutoPlugin {
     coverageOutputCobertura := true,
     coverageOutputDebug := false,
     coverageCleanSubprojectFiles := true,
-    coverageOutputTeamCity := false
+    coverageOutputTeamCity := false,
+    coveragePluginVersion := DefaultScoverageVersion
   )
 
   /**