Make sure systemd target is loaded before trying to stop it
Make sure systemd target is loaded before trying to stop it

During the initial deploy there is no systemd target to load which
means the command `systemctl stop` results in an error.

Now we check that the target exists before running `systemctl stop`.

The reason for stopping it in the first place is to make sure no
old processes are left behind, which could otherwise happend if
we remove/rename one of the processes in the procfile.
See https://github.com/twingly/capistrano-twingly/issues/31#issuecomment-486578861

close #49

twingly/capistrano-twingly