upgrade returns non-zero if already on latest version

I'm not sure this is really the proper fix, but it *does* prevent a chef run from completely failing.

This is roughly the same functionality that [existed before](https://github.com/pivotal-sprout/sprout-homebrew/commit/f71a372434d39114f04fb890069555f08d3530cd#diff-b3039281f18eb7df068fe2e6e099831c)

See [this issue](https://github.com/pivotal-sprout/sprout-homebrew/issues/5) for details.

Also note that [homebrew purposefully exits with non-zero in this case](https://github.com/Homebrew/homebrew/issues/27048)