Change Travis CI server sigil to matthagenbuch

- Needs Matt to add git hook for travis to enable
- Intended for pull request

@matthagenbuch to have this work:
* Navigate to: https://github.com/matthagenbuch/home-assistant/settings/hooks
* Select the "service" dropdown and find Travis CI
* Go to https://travis-ci.com/profile and sign in with github (may require you allow it as an app) and enable the switch for it

After you do that, accept this pull request and the icon in the readme will show the build status of YOUR repository.