Start building against Reactor Bismuth-SR4 snapshots

See gh-11030

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

@@ -127,7 +127,7 @@
 		<quartz.version>2.3.0</quartz.version>
 		<querydsl.version>4.1.4</querydsl.version>
 		<rabbit-amqp-client.version>5.0.0</rabbit-amqp-client.version>
-		<reactor-bom.version>Bismuth-SR3</reactor-bom.version>
+		<reactor-bom.version>Bismuth-BUILD-SNAPSHOT</reactor-bom.version>
 		<rest-assured.version>3.0.5</rest-assured.version>
 		<reactive-streams.version>1.0.1</reactive-streams.version>
 		<rxjava.version>1.3.3</rxjava.version>