Adjusted Travis' PHPUnit to use the vendor binary instead of a global install

.travis.yml

@@ -70,10 +70,9 @@ install:
     fi
   - composer install
   - |
-    # Install WP Test suite, install PHPUnit globally:
+    # Install WP Test suite:
     if [[ ! -z "$WP_VERSION" ]]; then
       bash tests/bin/install.sh woocommerce_test root '' localhost $WP_VERSION
-      composer global require "phpunit/phpunit=6.5.*|7.5.*"
     fi
 
 script:

tests/bin/phpunit.sh

@@ -5,7 +5,7 @@ if [[ ${RUN_PHPCS} == 1 ]] || [[ ${RUN_E2E} == 1 ]]; then
 fi
 
 if [[ ${RUN_CODE_COVERAGE} == 1 ]]; then
-	phpdbg -qrr $HOME/.composer/vendor/bin/phpunit -d memory_limit=-1 -c phpunit.xml --coverage-clover=coverage.clover --exclude-group=timeout $@
+	phpdbg vendor/bin/phpunit -d memory_limit=-1 -c phpunit.xml --coverage-clover=coverage.clover --exclude-group=timeout $@
 else
-	$HOME/.composer/vendor/bin/phpunit -c phpunit.xml $@
+	vendor/bin/phpunit -c phpunit.xml $@
 fi