Running e2e tests should stop a server in dev mode and start a server in prod mode

[jamsea]

Currently, running wildcat-protractor-runner will check if there's a server running on a specified port before starting one. However, if a server is already running in dev mode the e2e tests will run against the dev server instead of a prod one.

This can lead to subtle false positives and negatives, and is the main issue blocking https://github.com/nfl/react-wildcat/issues/142 from working 100% correctly. Instead, we should kill the process listening to the specified port in wildcat.config.js (default is 3000) and start a new one.