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

proxy/build.gradle

@@ -5,9 +5,14 @@ plugins {
     id 'checkstyle'
 }
 
+apply plugin: 'org.cadixdev.licenser'
 apply from: '../gradle/checkstyle.gradle'
 apply plugin: 'com.github.johnrengelman.shadow'
 
+license {
+    header = project.rootProject.file('HEADER.txt')
+}
+
 java {
     sourceCompatibility = JavaVersion.VERSION_1_8
     targetCompatibility = JavaVersion.VERSION_1_8