Don't use deprecated scalaJSUseRhino setting.

The warning says:
```
: warning: value scalaJSUseRhino in object AutoImport is deprecated: Will be removed in 1.0.0. Note that Rhino is not used by default anymore, so setting `scalaJSUseRhino` to `false` is redundant. To enable Rhino anew, use `Seq(Compile, Test).flatMap(c => inConfig(c)(jsEnv := RhinoJSEnv().value))`.
      scalaJSUseRhino in Test := true
      ^
```

After following the advice there is another warning:
```
: warning: method RhinoJSEnv in object AutoImport is deprecated: The Rhino JS environment is being phased out. It will be removed in Scala.js 1.0.0.
      inConfig(Test)(jsEnv := RhinoJSEnv().value)
                              ^
```

For now we must live with it.

Author: gslowikowski

build.sbt

@@ -73,7 +73,7 @@ lazy val runtime = CrossProject("scalac-scoverage-runtime", file("scalac-scovera
     .jsSettings(
       libraryDependencies += "org.scalatest" %%% "scalatest" % ScalatestVersion % "test",
       scalaJSStage := FastOptStage,
-      scalaJSUseRhino in Test := true
+      inConfig(Test)(jsEnv := RhinoJSEnv().value)
     )
 
 lazy val `scalac-scoverage-runtimeJVM` = runtime.jvm