Update SpringBoot to 2.2.0

This avoids an annoying warning (and some unhelpful exceptions thrown &
caught) on starting the app:

eclipse-ee4j/glassfish-hk2#462

but probably also pulls in some security & other fixes, and generally
staying up to date seems like a good plan if it doesn't break anything.

pom.xml

@@ -14,7 +14,7 @@
 	<parent>
 		<groupId>org.springframework.boot</groupId>
 		<artifactId>spring-boot-starter-parent</artifactId>
-		<version>2.1.5.RELEASE</version>
+		<version>2.2.0.RELEASE</version>
 		<relativePath/> <!-- lookup parent from repository -->
 	</parent>