fix test dependencies

Test data fixtures have been moved to the xabbuh/xapi-data-fixtures
package. Therefore, we need to depend on this package for tests.

Author: xabbuh

composer.json

@@ -18,7 +18,8 @@
         "symfony/validator": "~2.5"
     },
     "require-dev": {
-        "symfony/property-access": "~2.2"
+        "symfony/property-access": "~2.2",
+        "xabbuh/xapi-data-fixtures": "~1.0"
     },
     "autoload": {
         "psr-4": {