move test fixtures to the require section

The `StatementRepositoryTest` class which acts as the base class for
test classes in repository implementation packages depends on the test
fixtures package.

Author: xabbuh

composer.json

@@ -12,9 +12,7 @@
     ],
     "require": {
         "ramsey/uuid": "~2.8",
-        "php-xapi/model": "~0.1"
-    },
-    "require-dev": {
+        "php-xapi/model": "~0.1",
         "php-xapi/test-fixtures": "~0.1"
     },
     "conflict": {