Add the ability to run webdriver tests on already installed browsers on Win and Mac

[KaptenJansson]

Since travis-multirunner only supports linux (debian based) we could skip if Mac and Windows are detected and use already installed browsers.

This is better than not being able to run anything at all.

This is already possible today but it would be nice if we could do it automagically instead of having to modify the test framework code every time.