moves memcache mock dependency to require-dev (GoogleCloudPlatform#166)

Author: bshaffer

appengine/flexible/memcache/composer.json

@@ -1,10 +1,10 @@
 {
     "require": {
         "silex/silex": "^1.3",
-        "twig/twig": "^1.24",
-        "gecko-packages/gecko-memcache-mock": "^2.0"
+        "twig/twig": "^1.24"
     },
     "require-dev": {
+        "gecko-packages/gecko-memcache-mock": "^2.0",
         "google/cloud-tools": "<=1.0",
         "guzzlehttp/guzzle": "^6.2",
         "monolog/monolog": "^1.19",