Add trailing newline to JSON print

It is not needed in RStudio, becuse that
adds a newline before putting out a prompt. It is
needed in console R and Emacs, etc.

Before:
```r
> jsonlite::toJSON(1:4)
[1,2,3,4]> 
```

After:
```r
> toJSON(1:4)
[1,2,3,4] 
> 
```