Downgrade CaskAlreadyInstalledError to a warning

Automatic installs - like thoughtbot's laptop script - fail when we
use Homebrew's onoe method.  Using opoo (to warn) will let the
scripts complete successfully when they try to install packages that
have already been installed.

Test for downgrading AlreadyInstalledError to warning

Output of unit tests:
```
Finished in 79.407864s, 36.8603 runs/s, 79.6269 assertions/s.

2927 runs, 6323 assertions, 0 failures, 0 errors, 181 skips
```

Output of rspec:
```
Finished in 0.19466 seconds (files took 0.48031 seconds to load)
32 examples, 0 failures

Randomized with seed 17284
```