Simplify test harness by moving testing deps into devDependencies

Note: I based this on https://github.com/balderdashy/sails/pull/1432 to verify that Travis will still run tests after these changes.

Everything in `devDependencies` automatically gets installed with `npm install`, so this seems like an easy win to simplify the way the testing harness works. I could be missing some goal of having a custom script and field in `package.json`, though.