Improve printing output of `jill update`

[carstenbauer]

Currently, you only get

➜  ~/repos/jill.py git:(master) jill update
check new Julia release info, it will take a while

Would be great if it would also tell the user whether it has found new versions and perhaps even what the latest stable version is.

[johnnychen94]

closed by #58

@crstnbr if you're going to close multiple issues/PRs, you have to duplicate the close, e.g.,

close #58
close #18