Relicense the Velocity proxy under the terms of GPLv3 (#448)

native/build.gradle

@@ -4,6 +4,7 @@ plugins {
     id 'maven-publish'
 }
 
+apply plugin: 'org.cadixdev.licenser'
 apply from: '../gradle/checkstyle.gradle'
 apply from: '../gradle/publish.gradle'
 
@@ -12,6 +13,10 @@ java {
     targetCompatibility = JavaVersion.VERSION_1_8
 }
 
+license {
+    header = project.rootProject.file('HEADER.txt')
+}
+
 dependencies {
     implementation "com.google.guava:guava:${guavaVersion}"
     implementation "io.netty:netty-handler:${nettyVersion}"