Upgrade to SLF4J 1.7.16

Closes gh-5223

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -124,7 +124,7 @@
 		<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>1.7.15</slf4j.version>
+		<slf4j.version>1.7.16</slf4j.version>
 		<snakeyaml.version>1.16</snakeyaml.version>
 		<solr.version>4.10.4</solr.version>
 		<spock.version>1.0-groovy-2.4</spock.version>