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.
`source` is not POSIX, use `.`. There's some missing quoting here, though it's *highly* unlikely `$HOME` will contain spaces, so you may not care. We also say to reserve upper case names for environment variables in our guide.

```sh
local_hook="$HOME"/.git_template.local/hooks/post-checkout
[ -f "$local_hook ] && . "$local_hook"
```

sidenote: http://www.shellcheck.net/ is far better at this than I, for future reference.