Revert "Upgrade to Jackson 2.6.0"

This reverts commit f76197066c.

See gh-3569

spring-boot-dependencies/pom.xml

@@ -84,7 +84,7 @@
 		<httpclient.version>4.5</httpclient.version>
 		<httpcore.version>4.4.1</httpcore.version>
 		<infinispan.version>7.2.3.Final</infinispan.version>
-		<jackson.version>2.6.0</jackson.version>
+		<jackson.version>2.5.4</jackson.version>
 		<janino.version>2.7.8</janino.version>
 		<javassist.version>3.18.1-GA</javassist.version> <!-- Same as Hibernate -->
 		<javax-cache.version>1.0.0</javax-cache.version>