Add test:selenium task as prereq for test task
Add test:selenium task as prereq for test task

Now

1. There is a `rake test:selenium` task
2. It is a prerequisite for `rake test` and therefore for `rake`, ergo you can
   run the Selenium tests by simply running `rake`.

appfolio/selenium_test_demo_app,appfolio/selenium_test_demo_app,appfolio/selenium_test_demo_app