Change to Shields.io badge

I get that it's cooler to roll your own badge, however I don't think this is a good idea and here's why.

Every service that integrated with GitHub and it's repos had their own badge at some point: Code Climate had one, Ruby Gems had one, Travis CI and so on and so forth - however, this tends to look messy and inconsistent when put together into one README. This is also why I don't think PHPCI should roll their own.

Instead, services like Travis, Ruby Gems, Packagist and more have all moved on to use Shields.io for badges. It looks neat.

I would also like to phase out the build-status image in favor of only .svg's, because the images do not scale well (and shield.io does not provide them).