Improve the Travis configuration
Improve the Travis configuration

- switch to the faster Docker-based infrastructure given that we don't
  need to use sudo in the build
- cache the composer cache between builds, which is possible on the new
  infrasture, to be able to reuse previous downloads.
- switch to use archive downloads for stable dependencies. Composer is
  now able to fallback to source install when the github API rate limit
  is reached, and the usage of the cache should avoid hitting the github
  API for many downloads.

minkphp/MinkBrowserKitDriver