Upgrade to Appengine 1.9.53

Closes spring-projectsgh-9175

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -45,7 +45,7 @@
 	<properties>
 		<activemq.version>5.13.5</activemq.version>
 		<antlr2.version>2.7.7</antlr2.version>
-		<appengine.version>1.9.51</appengine.version>
+		<appengine.version>1.9.53</appengine.version>
 		<artemis.version>1.3.0</artemis.version>
 		<aspectj.version>1.8.10</aspectj.version>
 		<assertj.version>2.5.0</assertj.version>