Can't resolve PlayKeys.ws

There's an error in the migration doc for including Play WS. Following the advice to enable the WS plugin by adding "libraryDependencies += PlayKeys.ws" to my build.sbt I get the following error

/...../build.sbt:22: error: value ws is not a member of object play.Play.autoImport.PlayKeys
Merge pull request #2930 from GitsMcGee/master

Can't resolve PlayKeys.ws