only do selenium version checking if local binaries do not already exist

It was never my intention that this would check on every test run. If someone wants to stick to a specific version of selenium or chromedriver, they should be able to. If they want to make sure they are on the latest version, they can use the cli option `--update-selenium` or remove the module and reinstall it or remove `node_modules/testium/bin` and rerun their tests.

This allows offline test runs to NOT throw an error in stderr.