Shields: Add Depfu badges

[AlexWayfer]

Gemnasium will shut down, so I want to switch to Depfu, and I didn't found their badges in your service.

[chris48s]

Thanks for raising this. We'll also want to replace the existing Gemnasium integrations with an error message (refs #1449 ) when the service goes away on May 15th

[chris48s]

Having a quick look, depfu does not appear to have an API but they do appear to offer their own badges: https://github.com/depfu/example-ruby/blob/master/README.md

[AlexWayfer]

Having a quick look, depfu does not appear to have an API but they do appear to offer their own badges: https://github.com/depfu/example-ruby/blob/master/README.md

It looks like they added badges only 9 days ago.

I asked for API in their feedback repo.