Upgrade to Solr 6.6.4

Closes gh-13468

Author: wilkinsona

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

@@ -150,7 +150,7 @@
 		<simple-json.version>1.1.1</simple-json.version>
 		<slf4j.version>1.7.25</slf4j.version>
 		<snakeyaml.version>1.19</snakeyaml.version>
-		<solr.version>6.6.3</solr.version>
+		<solr.version>6.6.4</solr.version>
 		<spring.version>5.0.7.RELEASE</spring.version>
 		<spring-amqp.version>2.0.3.RELEASE</spring-amqp.version>
 		<spring-batch.version>4.0.1.RELEASE</spring-batch.version>