karma.conf.js - Switch singleRun back to `false` default

karma.conf.js - Switch singleRun back to `false` default

It's useful to have two ways of running the Karma test suite:

* For continuous integration, run once and report back with a file.
  (This has been `karma start --browsers PhantomJS --single-run --reporters dots,junit`.)
* For local dev, run in a watch/live/monitoring mode which auto-executes any time you edit the JS code.
  (This has been `karma start`)

This partially reverts a recent change so that the second one works the same
way as before.