Refactor logging dependencies for SBDG Starters Gradle build files.

Remove the exclude on 'org.apache.logging.log4j:log4j-to-slf4j'.

Add an exclude on 'org.apache.logging.log4j:log4j-core'.

Resolves gh-42.

spring-geode-starter-session/spring-geode-starter-session.gradle

@@ -7,6 +7,8 @@ dependencies {
 
     compile(project(':spring-geode-starter'))
 
-    compile 'org.springframework.session:spring-session-data-geode'
+    compile('org.springframework.session:spring-session-data-geode') {
+        exclude group: "org.apache.logging.log4j", module: "log4j-core"
+    }
 
 }