Upgrade to Tomcat 8.5.5

As part of the upgrade, Tomcat now requires a keystore to contain
an X.509 certificate. The two stores used in our tests have been
updated by exporting their private keys and adding them as
certificates. For example:

$ keytool -exportcert -keystore test.jks -alias tomcat > exported
$ keytool -importcert -keystore test.jks -file exported

Closes gh-6703
Closes gh-6657

spring-boot-dependencies/pom.xml

@@ -175,7 +175,7 @@
 		<thymeleaf-layout-dialect.version>1.4.0</thymeleaf-layout-dialect.version>
 		<thymeleaf-extras-data-attribute.version>1.3</thymeleaf-extras-data-attribute.version>
 		<thymeleaf-extras-java8time.version>2.1.0.RELEASE</thymeleaf-extras-java8time.version>
-		<tomcat.version>8.5.4</tomcat.version>
+		<tomcat.version>8.5.5</tomcat.version>
 		<undertow.version>1.3.23.Final</undertow.version>
 		<velocity.version>1.7</velocity.version>
 		<velocity-tools.version>2.0</velocity-tools.version>