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.

assets/scripts/app/models/build.coffee

@@ -45,7 +45,7 @@ require 'travis/model'
   ).property('jobs.@each.allowFailure')
 
   rawConfigKeys: (->
-    keys = Travis.Helpers.configKeys(@get('config'))
+    keys = []
 
     @get('jobs').forEach (job) ->
       Travis.Helpers.configKeys(job.get('config')).forEach (key) ->