update sbt-native-packager 0.7.2-RC1

refactoring build files
http://stackoverflow.com/a/24103453/605582
  [1]: http://stackoverflow.com/questions/23944108/how-to-share-version-values-between-project-plugins-sbt-and-project-build-scala/24103453#24103453

I recently shared this technique on Stackoverflow [How to share version values between project/plugins.sbt and project/Build.scala?][1]. Not sure if it's any different from Play depending on sbt-native-packager etc.