Start building against Spring Framework SNAPSHOTs

See gh-14481

Author: philwebb

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

@@ -157,7 +157,7 @@
 		<snakeyaml.version>1.23</snakeyaml.version>
 		<solr.version>7.4.0</solr.version>
 		<!-- deprecated in favor of "spring-framework.version" -->
-		<spring.version>5.1.0.RC3</spring.version>
+		<spring.version>5.1.0.BUILD-SNAPSHOT</spring.version>
 		<spring-amqp.version>2.1.0.M3</spring-amqp.version>
 		<spring-batch.version>4.1.0.M3</spring-batch.version>
 		<spring-cloud-connectors.version>2.0.3.RELEASE</spring-cloud-connectors.version>