feat(brew): add "mtr" to Brewfile
feat(brew): add "mtr" to Brewfile

Discovered this while troubleshooting my ISP blocking GitHub access:

https://twitter.com/wincent/status/1240236991154921472

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent