wireshark: use git repo for head

http://www.wireshark.org/lists/wireshark-dev/201401/msg00189.html

Closes #27119.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
Make conditional more obvious
reinstall: fix incorrect use of Formula.canonical_name

Fixes #27117.
reinstall: re-raise original exception after cleaning up
Compare pathnames directly
Only propagate universal option if the formula defines it
Update libpng download url
libpng: update 1.5.17 bottle.