Specify phpunit dev dependency in composer.json

And use it for the travis build.

.travis.yml

@@ -10,9 +10,11 @@ php:
   - nightly
   - hhvm
 
-install: composer install
+install: composer install --prefer-source
 
 matrix:
   allow_failures:
     - php: nightly
   fast_finish: true
+
+script: vendor/bin/phpunit

composer.json

@@ -13,6 +13,9 @@
         "php": ">=5.4",
         "ext-tokenizer": "*"
     },
+    "require-dev": {
+        "phpunit/phpunit": "~4.0"
+    },
     "autoload": {
         "psr-4": {
             "PhpParser\\": "lib/PhpParser"