Use POSIX compliant test command in git hooks

One of the students in Metis is running Ubuntu and `/bin/sh` is running as `Dash`. This is erroring out on `[[` in the `post-*` git hook files.

Switching to using POSIX compliant `[` command in place of the Bash specific `[[` per https://wiki.ubuntu.com/DashAsBinSh#A.5B.5B. @pbrisbin Can you take a look to verify my shell syntax is good?

Likely want to update https://github.com/thoughtbot/dotfiles/pull/299 as well if this gets merged in.
Lost some spaces here, though that might be subjected: `[ -f ... ]`.