Polish build matrix table header

Instead of uniformly camelizing the headers, use a map of headers
each build matrix key should display in the table
Fix typo [skip ci]
Remove 'commiter' key for translations

This key is not in use anywhere else.
Merge pull request #234 from travis-ci/ha-feature-gh234-pretty_build_key_headers

Build Matrix keys should be more flexible than just being camelized
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.