Add URL to project.clj

This makes it easier to find the code from Clojars.

My strategy is:

1. see thing I haven't heard of in someone's project.clj dependencies
2. go to https://clojars.org/thingname
3. click on the link to GitHub (or wherever)
4. now I know what that thing is!

Step 3 doesn't work without the URL being included.