Implemented --version-check

This adds a `--version-check` option to the console -- convenient to all package distributions (inc. Phar).

It uses tags from the main PHPUnit repo on Github (sebastianbergmann/phpunit) to extract the latest (and stable) version numbers to compare against.

I could move this code to `PHPUnit_Runner_Version` if it is preferable?

For the story, I started to implement `PHPUnit_Util_SelfUpdate` in **3.7** and  I was about to PR -- just to find out that **master** had a an implementation too... 
Anything from 10X to 30X are possibilities and may arise for one reason or another... 

For instance, Github may temporally alter things their end e.g. 307 (temp redirect), etc...
Local networks, caches, proxies may also get into play.