minor #398 Improve the Travis config (stof)
minor #398 Improve the Travis config (stof)

This PR was merged into the 1.3.x-dev branch.

Discussion
----------

Improve the Travis config

- use the faster container-based infrastructure
- use the build cache to keep the composer download cache

Commits
-------

46a0879 Improve the Travis config

hason/DoctrineBundle,nicolas-grekas/DoctrineBundle,gencer/DoctrineBundle,mikeSimonson/DoctrineBundle,Soullivaneuh/DoctrineBundle,chrisguitarguy/DoctrineBundle,mikeSimonson/DoctrineBundle,hason/DoctrineBundle,OskarStark/DoctrineBundle,OskarStark/DoctrineBundle,gencer/DoctrineBundle,javiereguiluz/DoctrineBundle,nicolas-grekas/DoctrineBundle,chrisguitarguy/DoctrineBundle,javiereguiluz/DoctrineBundle,skafandri/DoctrineBundle,hacfi/DoctrineBundle,hacfi/DoctrineBundle,Soullivaneuh/DoctrineBundle,fabpot/DoctrineBundle,fabpot/DoctrineBundle,fervo/DoctrineBundle,doctrine/DoctrineBundle,skafandri/DoctrineBundle,fervo/DoctrineBundle