Scala: prevent program output from trampling on REPL prompt

[raxod502]

Currently, this is what it looks like if you run the example Scala program and then type something into the REPL:

Welcome to Scala 2.11.12 (OpenJDK 64-Bit Server VM, Java 11.0.8).
Type in expressions for evaluation. Or try :help.

scala> Hello, world!
1 + 1
res1: Int = 2

scala> 

Notice that the REPL prompt appears before the program has finished execution, and therefore the console output looks wrong. This should be fixed, if possible.