Upgrade to Tomcat 8.0.20

Fixes gh-2547

Author: philwebb

spring-boot-dependencies/pom.xml

@@ -135,7 +135,7 @@
 		<thymeleaf-extras-conditionalcomments.version>2.1.1.RELEASE</thymeleaf-extras-conditionalcomments.version>
 		<thymeleaf-layout-dialect.version>1.2.7</thymeleaf-layout-dialect.version>
 		<thymeleaf-extras-data-attribute.version>1.3</thymeleaf-extras-data-attribute.version>
-		<tomcat.version>8.0.18</tomcat.version>
+		<tomcat.version>8.0.20</tomcat.version>
 		<undertow.version>1.1.1.Final</undertow.version>
 		<velocity.version>1.7</velocity.version>
 		<velocity-tools.version>2.0</velocity-tools.version>