Start building against Spring Framework 5.1.5 snapshots

See gh-15862

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -162,7 +162,7 @@
 		<snakeyaml.version>1.23</snakeyaml.version>
 		<solr.version>7.4.0</solr.version>
 		<!-- deprecated in favor of "spring-framework.version" -->
-		<spring.version>5.1.4.RELEASE</spring.version>
+		<spring.version>5.1.5.BUILD-SNAPSHOT</spring.version>
 		<spring-amqp.version>2.1.3.RELEASE</spring-amqp.version>
 		<spring-batch.version>4.1.1.RELEASE</spring-batch.version>
 		<spring-cloud-connectors.version>2.0.4.RELEASE</spring-cloud-connectors.version>