Get config headers only from jobs when preparing jobs table
Get config headers only from jobs when preparing jobs table

After changes in travis-core, irrelevant headers are removed when matrix
is expanded. For example python version is removed from a ruby build.
Build's config is not altered, so in order to get only effective keys,
we need to iterate over jobs.

fauxton/travis-web,fauxton/travis-web,Tiger66639/travis-web,travis-ci/travis-web,fauxton/travis-web,Tiger66639/travis-web,travis-ci/travis-web,jlrigau/travis-web,jlrigau/travis-web,fotinakis/travis-web,mjlambert/travis-web,jlrigau/travis-web,mjlambert/travis-web,2947721120/travis-web,2947721120/travis-web,travis-ci/travis-web,fauxton/travis-web,fotinakis/travis-web,jlrigau/travis-web,fotinakis/travis-web,fotinakis/travis-web,2947721120/travis-web,2947721120/travis-web,mjlambert/travis-web,Tiger66639/travis-web,travis-ci/travis-web,mjlambert/travis-web,Tiger66639/travis-web