improves bash code that verifies if homebrew is installed

The following pull request updates the code that verifies that homebrew is installed to be better. When I ran the command `make deps` I get this error,

```
$ make deps 
./tools/provision.sh
[+] detected mac os x
[!] could not find homebrew. please install it from http://brew.sh/
make: *** [deps] Error 1
```

I have brew installed in the directory `/opt/boxen/homebrew/bin/brew` which I found out via,

```
$ type brew                                     
brew is /opt/boxen/homebrew/bin/brew
```

Many people use boxen or install homebrew in locations other than `/usr/local/bin/brew` (which is currently hard coded).