Homebrew idempotent

This is my new pull request to replace [#155](https://github.com/thoughtbot/laptop/pull/155).

Converted my check for brew to a if ! type brew; then statement.

While I was in there I fixed the horked up injection of PATH to brew so you don't end up clobbering future additions to PATH.  

Result of fi is now the following gets injected into .zshrc (which I think is incorrect and you actually want .zshenv but that is a different discussion):

```
# recommended by brew doctor
export PATH="/usr/local/bin:$PATH"
```