You are doing everything right and it should work. But there was a bug in docker-compose 1.11.0 which not recognized file format 3.1 as valid. (3.0 works).
There is already a new release of docker-compose (1.11.1) which includes the bugfix:
Bugfixes
Fixed a bug where the 3.1 file format was not being recognized as valid by the Compose parser
So I would recommend to upgrade your docker-compose version if you want to use file format 3.1.
At the moment the safest way to upgrade docker-compose is by deleting it and reinstalling it.
rm /usr/local/bin/docker-compose
Reinstall:
curl -L https://github.com/docker/compose/releases/download/1.11.1/docker-compose-`uname -s`-`uname -m` > /usr/local/bin/docker-compose
chmod +x /usr/local/bin/docker-compose
docker-compose --version
docker-compose version 1.11.1, build 7c5d5e4
Now there isn’t an error anymore on the file format. (I did not test with your .yml).
docker-compose up
Starting compose_a_1
Starting compose_b_1