Require the correct version of phpunit depending on the PHP version.

Author: DaazKu

.travis.yml

@@ -15,9 +15,16 @@ branches:
         - upstream
 
 install:
-    - composer install
+    - composer self-update
+    - composer install -o
+    - |
+      if [[ ${TRAVIS_PHP_VERSION:0:2} == "7." ]]; then
+          composer require "phpunit/phpunit=5.7.*"
+      else
+          composer require "phpunit/phpunit=4.8.*"
+      fi
 
-script: phpunit -c phpunit.xml.dist --coverage-clover=coverage.clover
+script: ./vendor/bin/phpunit -c phpunit.xml.dist --coverage-clover=coverage.clover
 
 after_script:
     - wget https://scrutinizer-ci.com/ocular.phar