Switching to advertised composer-installed PHPUnit version

Especially relevant since PHPUnit versions change wildly depending on distro.
Let's use a version we all agree upon via `composer.json`

Author: Ocramius

.travis.yml

@@ -20,6 +20,7 @@ cache:
     - validation/frameworks
 
 before_script:
+  - composer install
   - set -e # Stop on first error.
   - phpenv config-rm xdebug.ini
   - if find . -name "*.php" -path "./src/*" -path "./experiments/*" -path "./tools/*" -path "./syntax-visualizer/server/src/*" -exec php -l {} 2>&1 \; | grep "syntax error, unexpected"; then exit 1; fi  
@@ -29,9 +30,9 @@ script:
   - composer validate
   - |
     if [[ $VALIDATION = true ]]; then
-      phpunit --testsuite validation
+      ./vendor/bin/phpunit --testsuite validation
     else
-      phpunit --testsuite invariants
-      phpunit --testsuite grammar
-      phpunit --testsuite api
+      ./vendor/bin/phpunit --testsuite invariants
+      ./vendor/bin/phpunit --testsuite grammar
+      ./vendor/bin/phpunit --testsuite api
     fi