INT-3741: Add Explicit spring-core Dependency

JIRA: https://jira.spring.io/browse/INT-3741

spring-retry pulls in an old version.

build.gradle

@@ -295,6 +295,7 @@ project('spring-integration-core') {
 	}
 
 	dependencies {
+		compile "org.springframework:spring-core:$springVersion"
 		compile "org.springframework:spring-aop:$springVersion"
 		compile "org.springframework:spring-context:$springVersion"
 		compile "org.springframework:spring-messaging:$springVersion"