Added some differences between Play 2.2 and sbt-less related to source maps.

Related discussion thread: https://groups.google.com/d/topic/play-framework/IO9BrHK4PN0/discussion

This is a backport of 18a36ed98c9923188152c60f37c5bb4fdc6ef019 to the 2.3.x branch.
Merge pull request #3053 from v6ak/2.3.x

[backport] Add some differences between Play 2.2 and sbt-less related to source maps