Add 'bundle exec' to the 'bin/qa' calls in qa/README.md

What does this MR do?

bundle exec is needed in order to load the Gemfile's gems otherwise you can get errors like the following:

     Gem::ConflictError:
       Unable to activate airborne-0.2.13, because rack-test-1.1.0 conflicts with rack-test (~> 0.6, >= 0.6.2)