[TASK] Use composer for unit and functional tests in travis

The patch uses composer based dependency handling and other
stuff pushing travis-ci tests to the next level:

- phpunit registered as "dev" requirement in composer.json
- vfsStream registered as "dev" requirement in composer.json
- cms-composer-installers is not released stable and adapted
  in composer.json for successful dependency resolving via
  composer
- cloning travis-integration repository is obsolete by inlining
  scripts in .travis.yml
- cloning ext:phpunit is obsolete, native phpunit by composer
  is used now
- it is possible to set functional test suite database credentials
  with environment variables, used in .travis.yml
- redis cache backend tests are enabled on travis-ci again
- use existing php modules of travis-ci environment speeds up build

With phpunit from composer, the command to run tests is
./bin/phpunit -c typo3/sysext/core/Build/UnitTests.xml
More documentation about that can be found at
http://wiki.typo3.org/Unit_Testing_TYPO3

Change-Id: I4d524e8e0249ca22f34fc48b7209d60d72e5e921
Resolves: #57971
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/29495
Reviewed-by: Wouter Wolters
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel