Add CLI command to open the UI and convert to using launchy gem

Converting to the launchy gem allows the browser to be opened across systems. (This lets open on start work for me on Debian)
Added an 'open' command on the CLI which will open the UI in the browser.

In the process of making this change I also refactored the Manager#start to extract the code for:
* starting all the runners into Runner.start_all
* waiting for the UI to start into Server.wait_for_start_and_open
Which simplifies the Manager#start code and gives the Manager class fewer things to be directly responsible for.

Finally I bumped the version to rc2