fix jumpToPDF, call displayline command line tools

TLDR: not all users have their apps installed in /Applications/.

True, apps from App Store live there.
But for Skim, we manually install it (may in anywhere), or install it
via Homebrew Cask (into /opt/homebrew-cask/Caskroom/skim/1.4.9).
For those people, it is a pain to find the cause then hack into this
file.

So, let symbolic link do the rest work.

1) For those use Homebrew Cask, Cask will set the link automatically.
2) For those installed Skim manually, use
    $ ln -s path/to/Skim.app/Contents/SharedSupport/displayline /usr/local/bin/displayline

It will also facilitate other applications.

It works well for me.
update README for symbolic link of displayline