README CI badge: explicitly mention branch

[elouanKeryell-Even]

README shows .

It refers to CI build of branch develop, but it doesn't explicitely says so, hence it can mislead user. For example, on a branch where CI does not pass, README will still show .

The idea is to be more explicit by replacing keyword build in the badge, by the name of the branch whose status is referred to: .

We have been using img.shields.io for that purpose (see this commit), but it is under nearly permanent denial of service since its infrastructure is undersized, letting us no choice but to revert (see this commit).

This comment by paulmelnikow, 2^nd contributor on badges/shields, mentions another service, badgen, that apparently does not suffer from denial of service.

We should use it, on this project and on all other projects too.

Badges in this issue are generated using https://badgen.now.sh/.

ping @sebbousquet

[elouanKeryell-Even]

Actually, nothing indicates that https://badgen.now.sh/ never goes under denial of service. However, I think its worth trying it, even if it means reverting if it ends up not working well.

[elouanKeryell-Even]

In fact, I noticed https://badgen.now.sh doesn't manage changing text of the travis-ci badge, hence it doesn't answer our need.