Upgrade to Slf4j 1.7.24

Closes gh-8450

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -141,7 +141,7 @@
 		<sendgrid.version>2.2.2</sendgrid.version>
 		<servlet-api.version>3.1.0</servlet-api.version>
 		<simple-json.version>1.1.1</simple-json.version>
-		<slf4j.version>1.7.23</slf4j.version>
+		<slf4j.version>1.7.24</slf4j.version>
 		<snakeyaml.version>1.17</snakeyaml.version>
 		<solr.version>5.5.4</solr.version>
 		<spock.version>1.0-groovy-2.4</spock.version>