All right! But this image don't control Travis builds, it's only displaying their status.
We must have the Travis status on each PR, Github displays it in the PR summary.

But, if you add this image as it in the "Readme" file, all branches will be have the same.
If you are working on a "feature-" branch, with a lot of work to do and the build failed, the "Readme" on the master branch will also show a "Failed build". But the code in master still safe and efficient.

Do you understand what I mean?