repl should add trailing newline to output when trying to print something

[onehr]

When trying to print something without trailing newline, there's no output in the terminal. But with trailing newline, it's fine.

[onehr]

I think the problem may occur because the line was flushed by `rustyline'.

[ffwff]

It is an issue with rustyline: https://github.com/kkawakam/rustyline/issues/216. When the patch gets published I'll update the dependency.