Command line use doesn’t exit with -n on completion

[davidaknowles]

JabRef version 4.0-beta3. Mac OS X 10.12.6

Why I care:
http://discourse.jabref.org/t/command-line-use-doesnt-exit-with-n-on-completion/798

Steps to reproduce:
java -jar JabRef.jar -n hangs indefinitely.

[MrTosz]

I'll have a go at it, if you don't mind.

[stefan-kolb]

Fixed in #3274