Start building against Spring AMQP 1.6.4 snapshots

1.6.4 snapshots are required for compatibility with Spring Retry 1.2

See gh-7158

spring-boot-dependencies/pom.xml

@@ -147,7 +147,7 @@
 		<solr.version>5.5.3</solr.version>
 		<spock.version>1.0-groovy-2.4</spock.version>
 		<spring.version>4.3.3.RELEASE</spring.version>
-		<spring-amqp.version>1.6.3.RELEASE</spring-amqp.version>
+		<spring-amqp.version>1.6.4.BUILD-SNAPSHOT</spring-amqp.version>
 		<spring-cloud-connectors.version>1.2.3.RELEASE</spring-cloud-connectors.version>
 		<spring-batch.version>3.0.7.RELEASE</spring-batch.version>
 		<spring-data-releasetrain.version>Ingalls-M1</spring-data-releasetrain.version>