Upgrade to thymeleaf-layout-dialect 1.2.8

Closes gh-2968

spring-boot-dependencies/pom.xml

@@ -132,7 +132,7 @@
 		<thymeleaf.version>2.1.4.RELEASE</thymeleaf.version>
 		<thymeleaf-extras-springsecurity3.version>2.1.1.RELEASE</thymeleaf-extras-springsecurity3.version>
 		<thymeleaf-extras-conditionalcomments.version>2.1.1.RELEASE</thymeleaf-extras-conditionalcomments.version>
-		<thymeleaf-layout-dialect.version>1.2.7</thymeleaf-layout-dialect.version>
+		<thymeleaf-layout-dialect.version>1.2.8</thymeleaf-layout-dialect.version>
 		<thymeleaf-extras-data-attribute.version>1.3</thymeleaf-extras-data-attribute.version>
 		<tomcat.version>8.0.22</tomcat.version>
 		<undertow.version>1.1.4.Final</undertow.version>