Merge pull request #254 from GoogleCloudPlatform/chore/php-version

Author: grayside

bookshelf/composer.json

@@ -1,6 +1,6 @@
 {
   "require": {
-    "php": ">=7",
+    "php": "^7.1.3",
     "laravel/lumen-framework": "5.8.*",
     "google/cloud-storage": "^1.11",
     "google/cloud-firestore": "^1.2",