Make whiskey a devDependency.

`whiskey` should be in `devDependencies`. `optionalDependencies` are always installed (if possible, failure is non-fatal), while `devDependencies` are not installed when the module is itself installed as a dependency. This is standard practice for test-runners.

If this is accepted, I'd like the same change in https://github.com/benjamn/install, making whiskey not get installed (twice!) with react-tools.