Move 'sandbox/scalajs/hello.scala' to 'sandbox/scalajs/src/main/scala/hello.scala' to fix an ensime error. After this change, sbt ensimeConfig works.

The ensime error message:
```
[error] You have .scala files in the base of your project. Such "script style" projects
[error] are not supported by ENSIME. Simply move them into src/main/scala to get support.
[error] Please read https://github.com/ensime/ensime-server/issues/1432
```

Author: ljdelight