Start building against Spring Framework 4.3.21 snapshots

See gh-15188

spring-boot-dependencies/pom.xml

@@ -153,7 +153,7 @@
 		<snakeyaml.version>1.17</snakeyaml.version>
 		<solr.version>5.5.5</solr.version>
 		<spock.version>1.0-groovy-2.4</spock.version>
-		<spring.version>4.3.20.RELEASE</spring.version>
+		<spring.version>4.3.21.BUILD-SNAPSHOT</spring.version>
 		<spring-amqp.version>1.7.11.RELEASE</spring-amqp.version>
 		<spring-cloud-connectors.version>1.2.6.RELEASE</spring-cloud-connectors.version>
 		<spring-batch.version>3.0.9.RELEASE</spring-batch.version>