Use SOURCE_VERSION env variable to get Heroku build commit

Since 1st April 2015, there's a `SOURCE_VERSION` environment variable
available to builds running on Heroku. For git-push builds, this is the
git commit SHA-1 of the source being built:

https://devcenter.heroku.com/changelog-items/630
http://stackoverflow.com/a/28718917/438886

app/controllers/Application.scala

@@ -110,25 +110,4 @@ object Application extends Controller {
         Ok(pullRequestSent(req.pr, req.user, mailType)).addingToSession(prId.slug -> toJson(updatedSettings).toString)
       }
   }
-
-  lazy val gitCommitId = gitCommitIdFromHerokuFile.getOrElse(app.BuildInfo.gitCommitId)
-
-  def gitCommitIdFromHerokuFile: Option[String]  = {
-    val existingFileOpt: Option[File] = herokuMetadataFile()
-
-    Logger.debug(s"Heroku dyno metadata: $existingFileOpt")
-
-    for {
-      existingFile <- existingFileOpt
-      commitId <- (Json.parse(new FileInputStream(existingFile)) \ "release" \ "commit").asOpt[String]
-    } yield {
-      Logger.debug(s"Heroku dyno commit id: $commitId")
-      commitId
-    }
-  }
-
-  def herokuMetadataFile(): Option[File] = {
-    val file = new File("/etc/heroku/dyno")
-    if (file.exists && file.isFile) Some(file) else None
-  }
 }

app/views/fragments/footer.scala.html

@@ -10,4 +10,4 @@
         <p></p>
     </div>
 </footer>
-<!-- @Application.gitCommitId -->
+<!-- @app.BuildInfo.gitCommitId -->

build.sbt

@@ -12,7 +12,7 @@ lazy val root = (project in file(".")).enablePlugins(
 ).settings(
   buildInfoKeys := Seq[BuildInfoKey](
     name,
-    BuildInfoKey.constant("gitCommitId", Option(System.getenv("BUILD_VCS_NUMBER")) getOrElse(try {
+    BuildInfoKey.constant("gitCommitId", Option(System.getenv("SOURCE_VERSION")) getOrElse(try {
       "git rev-parse HEAD".!!.trim
     } catch { case e: Exception => "unknown" }))
   ),