first attempt of travis usage...

Should be with --prefer-source, see https://travis-ci.org/hostnet/form-twig-bridge/jobs/27388329
```
 - Installing symfony/property-access (v2.5.0)
Downloading: connection...Failed to download symfony/property-access from dist: The "https://api.github.com/repos/symfony/PropertyAccess/zipball/1df395afb2208eb2989f605771ee7ea6cba1c775" file could not be downloaded (HTTP/1.1 403 Forbidden)
Now trying to download from source
```