add cli --update-selenium option

This PR implements an option for the command-line tool that will update the currently-used version of selenium standalone server and chromedriver. The only way to do this before this PR is to remove the `./node_modules/testium/bin` directory and try to run more tests.

```
$ ./node_modules/.bin/testium --update-selenium
[testium] grabbing selenium standalone server 2.39.0
[testium] grabbing selenium chromedriver 2.8
Up to date!
```

You also get this!
```
$ ./node_modules/.bin/testium --help
Usage: node ./node_modules/.bin/testium [browser|--updateSelenium|--help]
```