Add support for requires.io

[Wilfred]

It would be great to see support for requires.io on shields.io. Here's an example:

https://requires.io/github/Wilfred/trifle/requirements.png?branch=master

[espadrine]

I sent them an email on June 19th about that. Their response was positive:

We've been having quite a few requests from our users for shields.io
badges. It's pretty high on our backlog, so you will be hearing from
us soon!

[espadrine]

requires.io has added Shields badges!

They have also added an API so that we can fetch the necessary data from them.

(The following is an excerpt from the mail I received from Alexis Tabary.)

Example for the repository cool-RR/python_toolbox on GitHub:

• requires.io page: https://requires.io/github/cool-RR/python_toolbox/requirements/?branch=master
• API call: https://requires.io/api/v1/status/github/cool-RR/python_toolbox/?branch=master
• for a tag instead of a branch: https://requires.io/api/v1/status/github/cool-RR/python_toolbox/?tag=0.6.11
  (to get json just make a call with the header "accept: application/json" or add "&format=json" to the url)

There are four values possible for the status:

• up-to-date
• outdated
• insecure
• unknown

Colors should be like this:

• up-to-date -> brightgreen
• outdated -> yellow
• insecure -> red
• unknown -> lightgrey

(End excerpt.)

They did things really well!

If anyone wants to take a stab at implementing support in Shields, they are welcome to do so.

[espadrine]

Fixed by 7b830ca.