REPL: Tread EOF a "no" in the "yes"/"no" prompt.

Before, we got in an endless loop if using ^D to try to end the
session. When piping commands into the REPL, this was rather annoying!

```
 scala-hash v2.11.5
Welcome to Scala version 2.11.5-20150101-184742-3fafbc204f (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_25).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :require xxx
java.lang.NullPointerException
	at scala.tools.nsc.interpreter.ILoop.scala$tools$nsc$interpreter$ILoop$$flatten$1(ILoop.scala:651)
	at scala.tools.nsc.interpreter.ILoop.require(ILoop.scala:654)

That entry seems to have slain the compiler.  Shall I replay
your session? I can re-run each line except the last one.
[y/n]^D
You must enter y or n.
That entry seems to have slain the compiler.  Shall I replay
your session? I can re-run each line except the last one.
[y/n]^D
...

```

Author: retronym

src/repl/scala/tools/nsc/interpreter/InteractiveReader.scala

@@ -23,6 +23,7 @@ trait InteractiveReader {
   def readYesOrNo(prompt: String, alt: => Boolean): Boolean = readOneKey(prompt) match {
     case 'y'  => true
     case 'n'  => false
+    case -1   => false // EOF
     case _    => alt
   }