Make Jackson optional in web adapter

There is now a JsonMapper strategy with implementations and
autoconfiguration for Gson and Jackson. If Jackson is present
it is preferred (just like in Spring Boot).

Fixes gh-150

spring-cloud-function-web/pom.xml

@@ -23,6 +23,11 @@
 			<groupId>com.google.code.gson</groupId>
 			<artifactId>gson</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>com.fasterxml.jackson.core</groupId>
+			<artifactId>jackson-databind</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.cloud</groupId>
 			<artifactId>spring-cloud-function-context</artifactId>