improve Travis CI configuration

[xabbuh]

• fix the Composer cache directory
• run jobs with different Symfony versions only on PHP 5.6
• drop jobs for Symfony versions that reached EOL
• add job for PHP 7
• run code coverage reports only on PHP 7
• disable Xdebug when code coverage reports won't be generated
• finish as soon as possible (when either a mandatory job failed or only
  jobs that are allowed to fail are running)

[scrutinizer-notifier]

The inspection completed: 28 Issues, 1 Patches

[cryptiklemur]

Is it worth removing 2.6? Its EOLed in less than a month

[cryptiklemur]

by the time @kmfk merges this, it'll be EOLed ;)

[k-k]

@xabbuh - Thanks for this! I tried rebuilding your failed tests after merging #87 - but I still get the same error in Travis.

There are also tests failing where we previously checked for PHPUnit_Framework_Errors - but are now properly throwing TypeErrorExceptions in PHP 7. (see here).

Maybe create new tests for the correct exceptions and group them in PHPUnit so they can be ignored on the other tests - or use version_compare() inside the test to expect the correct exception?

[k-k]

Rebuilding this again, the YAML fix did actually work. I'm gonna merge this and fix the other tests in a separate PR.