Unable to get sbt-rjs incremental execution to work without clean

I'm trying to use sbt-rjs in a non-play sbt project.

My `plugins.sbt` includes this:
```scala
addSbtPlugin("com.typesafe.sbt" % "sbt-rjs" % "1.0.0-RC3")
```

And my `build.sbt` includes this:
```scala
import RjsKeys._
import WebJs._

lazy val root = (project in file(".")).enablePlugins(SbtWeb)

JsEngineKeys.engineType := JsEngineKeys.EngineType.Node

pipelineStages := Seq(rjs)
```

The first time I invoke `sbt web-stage` everything works perfectly, but on subsequent re-runs nothing happens. The only way I can get the pipeline to run again is if I remove the resulting artifacts from my target directory. So `sbt ~web-stage` won't work, I have to do something like `~;clean; web-stage`.