Test library against released php 7.2 (#84)

Author: chingor13

cloudbuild.yaml

@@ -9,7 +9,7 @@ steps:
     waitFor: ['-']
     id: php70
   - name: gcr.io/cloud-builders/docker
-    args: ['build', '--build-arg', 'BASE_IMAGE=gcr.io/php-mvm-a/php72:alpha3', '.']
+    args: ['build', '--build-arg', 'BASE_IMAGE=gcr.io/google-appengine/php72', '.']
     waitFor: ['-']
     id: php72
   - name: gcr.io/cloud-builders/docker

ext/README.md

@@ -20,7 +20,7 @@ This extension has been built and tested on the following PHP versions:
 
 * 7.0.x
 * 7.1.x
-* 7.2.0 (alpha3)
+* 7.2.x
 
 ## Installation