As for the current version 0.11.2 (2014-03-25), the way to define the merge strategy is different.
This is documented here, the relevant part is:
NOTE:
mergeStrategy in assembly expects a function, you can’t domergeStrategy in assembly := MergeStrategy.first
The new way is (copied from the same source):
mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
{
case PathList("javax", "servlet", xs @ _*) => MergeStrategy.first
case PathList(ps @ _*) if ps.last endsWith ".html" => MergeStrategy.first
case "application.conf" => MergeStrategy.concat
case "unwanted.txt" => MergeStrategy.discard
case x => old(x)
}
}
This is possibly applicable to earlier versions as well, I don’t know exactly when it has changed.