[example] dropwizard uses Gradle packaging

dd-trace-examples/dropwizard-mongo-client/dropwizard-mongo-client.gradle

@@ -1,12 +1,13 @@
-plugins {
-  id "com.github.johnrengelman.shadow" version "2.0.1"
-}
-
+apply plugin: 'application'
 apply from: "${rootDir}/gradle/java.gradle"
 apply from: "${rootDir}/gradle/jacoco.gradle"
 
 version = 'demo'
 description = 'dropwizard-mongo-client'
+mainClassName = 'com.datadoghq.example.dropwizard.BookApplication'
+
+sourceCompatibility = 1.8
+targetCompatibility = 1.8
 
 dependencies {
   compile project(':dd-trace-annotations')
@@ -19,16 +20,6 @@ dependencies {
   compile group: 'com.squareup.okhttp3', name: 'okhttp', version: '3.7.0'
 }
 
-jar {
-  manifest {
-    attributes 'Main-Class': 'com.example.helloworld.HelloWorldApplication'
-  }
-}
-
-shadowJar {
-  mergeServiceFiles()
-}
-
 task wrapper(type: Wrapper) {
   gradleVersion = '4.0'
 }