Start building against Spring Framework 5.1.8 snapshots

See gh-16961

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

@@ -162,7 +162,7 @@
 		<snakeyaml.version>1.23</snakeyaml.version>
 		<solr.version>7.7.1</solr.version>
 		<!-- deprecated in favor of "spring-framework.version" -->
-		<spring.version>5.1.7.RELEASE</spring.version>
+		<spring.version>5.1.8.BUILD-SNAPSHOT</spring.version>
 		<spring-amqp.version>2.1.6.RELEASE</spring-amqp.version>
 		<spring-batch.version>4.1.2.RELEASE</spring-batch.version>
 		<spring-cloud-connectors.version>2.0.5.RELEASE</spring-cloud-connectors.version>