Added codeception/specify to require-dev (#72)

It was an indirect dependency and #57 made Codeception builds with --prefer-lowest fail: https://travis-ci.org/Codeception/Codeception/jobs/145152589#L1653

composer.json

@@ -23,7 +23,8 @@
         "yiisoft/yii2-codeception": "*",
         "yiisoft/yii2-debug": "*",
         "yiisoft/yii2-gii": "*",
-        "yiisoft/yii2-faker": "*"
+        "yiisoft/yii2-faker": "*",
+        "codeception/specify": "*"
     },
     "config": {
         "process-timeout": 1800