Merge pull request heroku#40 from xuwei-k/sbt0.12.3

add sbt-0.12.3 support

opt/sbt-0.12.3.boot.properties

@@ -0,0 +1,20 @@
+[scala]
+  version: ${sbt.scala.version-auto}
+
+[app]
+  org: ${sbt.organization-org.scala-sbt}
+  name: sbt
+  version:  ${sbt.version-read(sbt.version)[0.12.3]}
+  class: ${sbt.main.class-sbt.xMain}
+  components: xsbti,extra
+  cross-versioned: ${sbt.cross.versioned-false}
+
+[repositories]
+  local
+  maven-local
+  heroku-typesafe-ivy: http://s3pository.heroku.com/ivy-typesafe-releases/, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext]
+  heroku-central: http://s3pository.heroku.com/maven-central/
+  heroku-typesafe-snapshots: http://s3pository.heroku.com/ivy-typesafe-snapshots/, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext]
+
+[log]
+ level: info