Improve the Travis configuration
Improve the Travis configuration

- switch to the container-based setup
- persist the Composer download cache between builds
- add testing on PHP 7
- test against LTS versions of Symfony rather than against unmaintained
  ones
- avoid notifying Scrutinizer of missing code coverage for PHP 7 and
  HHVM to let other jobs upload it

stof/Gherkin,higrow/Gherkin,Behat/Gherkin