Prefer 'missing config' error message to 'github pages branch'

Fixes travis-ci/travis-ci#5933

If `.travis.yml` contains a YAML syntax error on the `gh-pages`
branch, we display the error message "github pages branch" even if
`gh-pages` is (textually) whitelisted in that file.

We should display "missing config" instead.

Author: BanzaiMan

lib/travis/model/request/approval.rb

@@ -68,10 +68,10 @@ def message
         'skipped through commit message'
       elsif disabled_in_settings?
         request.pull_request? ? 'pull requests disabled' : 'pushes disabled'
-      elsif github_pages?
-        'github pages branch'
       elsif request.config.blank?
         'missing config'
+      elsif github_pages?
+        'github pages branch'
       elsif !branch_approved? || !branch_accepted?
         'branch not included or excluded'
       elsif !config_accepted?