Upgrade to Spring Boot 1.5.9

pom.xml

@@ -8,7 +8,7 @@
     <parent>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-parent</artifactId>
-        <version>1.5.8.RELEASE</version>
+        <version>1.5.9.RELEASE</version>
     </parent>
     <description>This application shows a few key concepts of
         Domain Driven Design implemented in Enterprise Java.