Explicitly declare Logback as the logging provider.

Add the 'spring-geode-starter-test' module as a testRuntime dependency in order to write Unit and Integration Tests for this 'spring-geode-starter-logging' module.

Resolves gh-73.

spring-geode-starter-logging/spring-geode-starter-logging.gradle

@@ -5,7 +5,10 @@ description = "Spring Boot Logging Starter for Apache Geode with Logback as the
 
 dependencies {
 
+    compile "ch.qos.logback:logback-classic"
     compile "org.apache.logging.log4j:log4j-to-slf4j"
     compile "org.springframework.boot:spring-boot-starter-logging"
 
+    testRuntime project(":spring-geode-starter-test")
+
 }