Upgrade to Snakeyaml 1.18

Closes spring-projectsgh-8505

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -151,7 +151,7 @@
 		<servlet-api.version>3.1.0</servlet-api.version>
 		<simple-json.version>1.1.1</simple-json.version>
 		<slf4j.version>1.7.24</slf4j.version>
-		<snakeyaml.version>1.17</snakeyaml.version>
+		<snakeyaml.version>1.18</snakeyaml.version>
 		<solr.version>5.5.4</solr.version>
 		<spock.version>1.0-groovy-2.4</spock.version>
 		<spring.version>5.0.0.BUILD-SNAPSHOT</spring.version>