Update docs badge to Inch CI

Hi there,

you told me last week that I could recruit you as an early adopter for my new Inch CI project, which allows you to refresh the badge yourself [on the page](http://inch-ci.org/github/jdantonio/concurrent-ruby) (you can even [configure a webhook to refresh](http://inch-ci.org/howto/webhook) auto-magically).

I did not yet submit this to any mailing lists or twitter or anything, but sooner rather than later Inch CI will replace Inch Pages. So if you would like to be one of the new site's first users, I would more than appreciate that!

Let me know what you think! :man: 

P.S. Note there are several links across the site to [open an issue on GitHub](https://github.com/inch-ci/inch_ci-web/issues/new) for any inconvenience you, as an early adopter, may come across.