How make travis-ci execute for some branch?

You can try and check if it would work with a whitelist.
See “Specify branches to build”

You can either white- or blacklist branches that you want to be built:

# blacklist
branches:
  except:
    - legacy
    - experimental

# whitelist
branches:
  only:
    - master
    - stable

Try and put A in a whitelist syntax in the travis.yml, and see if that is picked-up.