Why does my symbolic link not work?
I figured out what went wrong:
I made a typographical mistake when I was creating the symbolic link!

WRONG

sudo ln -s /Applications/Julia-0.6.app/Contents/Resources/bin/julia /usr/local/bin/julia


CORRECT

sudo ln -s /Applications/Julia-0.6.app/Contents/Resources/julia/bin/julia /usr/local/bin/julia


(I was missing the julia in .../Resources/julia/bin/...)