Preparing for PHPUnit 6

https://github.com/sebastianbergmann/phpunit/wiki/Preparing-for-PHPUnit-6

Author: SergeyZ

.travis.yml

@@ -20,7 +20,7 @@ env:
   - PHPUNIT=5.5.*     # EOL February 2, 2018
   - PHPUNIT=5.6.*     # EOL February 2, 2018
   - PHPUNIT=5.7.*     # EOL February 2, 2018
-  - PHPUNIT=6.0.x-dev # EOL February 2, 2018
+  - PHPUNIT=^6.0      # EOL February 2, 2018
 
 matrix:
   exclude:
@@ -36,6 +36,7 @@ cache:
 before_script:
   - phpenv config-rm xdebug.ini || true
   - composer self-update
+  - composer config minimum-stability dev
   - composer require phpunit/phpunit:${PHPUNIT}
 
 script: