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
PlayKeys.ws,
       ^
[error] Type error in expression
```

Looking at the source for PlayImport.scala, it seems apparent why. The ws value is not under the scope of the PlayKeys object, but rather a level higher, and indeed simply changing my build.sbt to "libraryDependencies += ws" solved the problem.