diff --git a/composer.json b/composer.json
index 2d27262..6072c36 100644
--- a/composer.json
+++ b/composer.json
@@ -4,14 +4,16 @@
     "license": "Apache-2.0",
     "minimum-stability": "stable",
     "require": {
-        "php": ">=5.5.9",
+        "php": ">=5.5",
         "rize/uri-template": "~0.3",
         "google/auth": "^0.11",
         "guzzlehttp/guzzle": "^5.3|^6.0",
         "guzzlehttp/psr7": "^1.2",
         "monolog/monolog": "~1",
-        "psr/http-message": "1.0.*",
-        "symfony/lock": "dev-master"
+        "psr/http-message": "1.0.*"
+    },
+    "suggest": {
+        "symfony/lock": "Required for the Spanner cached based session pool. Please require the following commit: dev-master#1ba6ac9"
     },
     "extra": {
         "component": {