Run Travis CI on container-based infrastructure.
Run Travis CI on container-based infrastructure.

Attempt to run Travis CI on container-based infrastructure to enable Composer cache, which is only available for private repositories and Travis' new container-based infrastructure.

See:
 * [Caching Dependencies and Directories](https://docs.travis-ci.com/user/caching)
 * [Routing your build to container-based infrastructure](https://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure)

SURFnet/yubikey-api-client-bundle