README: git bisect improvements

* further explain `git bisect` initialization
* belabor `git bisect start` a bit
always use first person
small grammar/spelling fixes
highlight the point `git bisect` succeeds
Remove TravisCI build

If we had CI while developing, we would have much more easily known
which commit introduced the bug because presumably we'd have already
been running tests after each commit!
Update README.md
main.c: make comments smaller

So everything fits on-screen during my slideshow.