Fixing a broken install URL and making the recipes more idempotent

So apparently the install link was broken, so I fixed that.

Additionally, and this one is personal I guess, but the default recipe always executes a brew update, which is perhaps not what you want. I know I personally only want my chef run executing something if there has been some intrinsic state change that requires something to be executed. If I really wanted to run "brew update" all the time, I could just include the `homebrew::update` recipe as well, but at least I'd have the choice.

Thoughts?