Restart w/ the new `passenger-config restart-app` command when available.

* Updates Guard::Passenger::Runner such that if the user is running a
	version of Passenger that includes the new `passenger-config restart-app`
	command, the runner will prefer restarting that way to touching
	tmp/restart.txt.
* Checks for an installed Passenger using Gem.latest_spec_for, to make it
	possible to distinguish between versions.
* Adds (passing!) tests for the above behaviors.
* Adds development dependency on RSpec::Mocks to help stub out Passenger
	versions.
* Updates and loosens one or two development dependencies, just a hair.
* Switches the Gemfile's old and deprecated `Config` module call to the
	newer `RbConfig`.
Update Changelog and bump version number
Merge branch 'phyllisstein-faster-restart'