add additionalImports for routes hash seed

#### current behavior of RoutesCompiler

1. compile all files
2. [`routesImport`](https://github.com/playframework/Play20/blob/2.1.0/framework/src/sbt-plugin/src/main/scala/PlayKeys.scala#L49) Key **chenge**
3. reload sbt console
4. compile
5. do **NOT** recompile routes. use wrong old cache. (should be recompile)