Include the correct version of phpunit.

Author: octalmage

.travis.yml

@@ -19,6 +19,14 @@ before_script:
   - composer install
   - npm install
   - bash tests/bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION
+  - export PATH="$HOME/.composer/vendor/bin:$PATH"
+  - |
+    if [[ ${TRAVIS_PHP_VERSION:0:2} == "7." ]]; then
+      composer global require "phpunit/phpunit=5.7.*"
+    else
+      composer global require "phpunit/phpunit=4.8.*"
+    fi
+
 
 script: 
   - phpunit