Add Travis-CI -configuration

See: https://travis-ci.org/datakurre/navigator-proto/builds

This pull requires a few extra actions:
- enable Travis-CI-hook for the repository (log into Travis-CI, sync repositories and enable this repository)
- update the address of the build status image at README
- for SauceLabs, replace encrypted SAUCE_ACCESS_KEY environment variable

```bash
gem install travis
travis encrypt SAUCE_ACCESS_KEY=mysecretaccesskey -r codeforeurope/navigator-proto --add env.global
```

Yet, SauceLabs tests are disabled in `.travis.yml` because the current integration seems to be too fragile (SauceLabs browser had a connection issue with Travis-CI, the issue with the first browser blocked testing the other, also it's missing support for "Sauce Labs tunnel identifiers", which prevents simultaneous tests or testing simultaneously on Travis-CI and on local machine). Waiting for a better SauceLabs-integration for testem...