Fix mirb to use readline

Current mirb can't interpret a code block across multiple lines if ENABLE_READLINE macro was defined.

current 

```
> def foo
*   "foo"
line 2: syntax error, unexpected tSTRING_BEG, expecting ';' or '\n'
>
```

fixed

```
> def foo
*   "foo"
* end
 => nil
> foo
 => "foo"
>
```