Fix empty newline errors in REPL

Just press enter to create some newlines. Works with `node` and `coffee`, not with `_node` or `_coffee`.

Before:

```
$ _node
_node> 
Error: error streamlining source: :2:missing operand on line 2
    at Object.exports.transform (/Users/aseemk/Projects/Node/streamlinejs/lib/callbacks/transform.js:1630:10)
    at REPLServer.evaluate [as eval] (/Users/aseemk/Projects/Node/streamlinejs/lib/compiler/repl.js:50:34)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)
    at ReadStream.EventEmitter.emit (events.js:98:17)
    at emitKey (readline.js:1095:12)
_node>
```

After:

```
$ _node
_node> 
undefined
_node> 
```