Fix directory test when cleaning existing Homebrew taps

OMERO-homebrew-stable has been failing since the Homebrew taps folder was successfully cleaned and homebrew-alt PR weren't merged in. From the [console]((http://hudson.openmicroscopy.org.uk/view/2.%20Stable/job/OMERO-homebrew-stable/OSX=10.6.8/103/console) of #103

```
++ '[-d' '/usr/local/Library/Taps]'
docs/hudson/OMERO-homebrew-install.sh: line 44: [-d: command not found
+++ curl -fsSL https://raw.github.com/mxcl/homebrew/go
```

Commit e2e8271 should fix this `command not found` error.