Upgraded Gradle Shadow plugin to 4.0.4 (last Gradle 4.x version)

proxy/build.gradle

@@ -1,6 +1,5 @@
 plugins {
     id 'java'
-    id 'de.sebastianboegl.shadow.transformer.log4j' version '2.1.1'
     id 'checkstyle'
 }
 
@@ -20,6 +19,10 @@ jar {
     }
 }
 
+shadowJar {
+    transform(com.github.jengelman.gradle.plugins.shadow.transformers.Log4j2PluginsCacheFileTransformer)
+}
+
 tasks.withType(Checkstyle) {
     exclude('**/com/velocitypowered/proxy/protocol/packet/*.java')
 }