Tudo funciona bem com o Play 2.2.1. No entanto, ao atualizar para o Play 2.2.2 e ao Heroku, o processo alerta sobre:

sbt.ResolveException: unresolved dependency: com.typesafe.play#play_2.10;2.2.2: not found
       unresolved dependency: com.typesafe.play#play-cache_2.10;2.2.2: not found
       unresolved dependency: com.typesafe.play#filters-helpers_2.10;2.2.2: not found
       unresolved dependency: com.typesafe.play#play-test_2.10;2.2.2: not found
       unresolved dependency: com.typesafe.play#play-docs_2.10;2.2.2: not found

Eu não entendo o porquê .. Acabei de atualizar o plugins.sbt para:

// Comment to get more information during initialization
logLevel := Level.Warn

// The Typesafe repository
resolvers += "Typesafe repository" at "http://repo.typesafe.com/typesafe/releases/"

// Use the Play sbt plugin for Play projects
addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.2.2") //just added 2.2.2 instead of 2.2.1

e sbt atualizado para 0.13.0.

Essa dependência não deve ser baixada sem um conjunto explícito no Build.scala?