fix(config): Make all file paths in config files relative to the config file itself

Breaking Change
Previously, onPrepare and specs were relative to the location of the config,
but seleniumServerJar and chromeDriver were relative to the cwd when the
test was called. If you were calling the tests from somewhere other than
the same directory as the config location, you will need to change the paths of
seleniumServerJar and/or chromeDriver.  Closes #222.