Update to GAE 1.9.76

Fixes gh-7606

Author: rwinch

gradle/dependency-management.gradle

@@ -47,11 +47,11 @@ dependencyManagement {
 		dependency 'com.fasterxml.jackson.core:jackson-databind:2.10.0'
 		dependency 'com.fasterxml:classmate:1.3.4'
 		dependency 'com.github.stephenc.jcip:jcip-annotations:1.0-1'
-		dependency 'com.google.appengine:appengine-api-1.0-sdk:1.9.76'
-		dependency 'com.google.appengine:appengine-api-labs:1.9.76'
-		dependency 'com.google.appengine:appengine-api-stubs:1.9.76'
-		dependency 'com.google.appengine:appengine-testing:1.9.76'
-		dependency 'com.google.appengine:appengine:1.9.76'
+		dependency 'com.google.appengine:appengine-api-1.0-sdk:$gaeVersion'
+		dependency 'com.google.appengine:appengine-api-labs:$gaeVersion'
+		dependency 'com.google.appengine:appengine-api-stubs:$gaeVersion'
+		dependency 'com.google.appengine:appengine-testing:$gaeVersion'
+		dependency 'com.google.appengine:appengine:$gaeVersion'
 		dependency 'com.google.code.gson:gson:2.8.2'
 		dependency 'com.google.guava:guava:20.0'
 		dependency 'com.google.inject:guice:3.0'