Add spring-data-commons optional dependency

Otherwise, the XSD cannot find spring-repository.

Issue gh-32

core/build.gradle

@@ -15,6 +15,7 @@ dependencies {
              "org.slf4j:slf4j-api:$slf4jVersion"
 
     optional "org.springframework:spring-context:$springVersion",
+            "org.springframework.data:spring-data-commons:$springDataVersion",
             "org.springframework:spring-jdbc:$springVersion",
             "org.springframework:spring-orm:$springVersion",
             "com.querydsl:querydsl-core:$queryDslVersion",