Fix CoverageFileName

fixes #135 

This issue is making builds using the Gradle BuildCache to report 0% coverage.
Gradle caches the .xml file that doesn't exist from the compile task and then the report task doesn't see anything, now it will cache the right file fixing the builds.

src/main/groovy/org/scoverage/ScoveragePlugin.groovy

@@ -191,7 +191,7 @@ class ScoveragePlugin implements Plugin<PluginAware> {
                 }
                 scalaCompileOptions.additionalParameters = parameters
                 // the compile task creates a store of measured statements
-                outputs.file(new File(extension.dataDir.get(), 'scoverage.coverage.xml'))
+                outputs.file(new File(extension.dataDir.get(), 'scoverage.coverage'))
 
                 dependsOn project.configurations[CONFIGURATION_NAME]
                 doFirst {