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

api/build.gradle

@@ -4,6 +4,7 @@ plugins {
     id 'checkstyle'
 }
 
+apply plugin: 'org.cadixdev.licenser'
 apply from: '../gradle/checkstyle.gradle'
 apply from: '../gradle/publish.gradle'
 apply plugin: 'com.github.johnrengelman.shadow'
@@ -13,6 +14,10 @@ java {
     targetCompatibility = JavaVersion.VERSION_1_8
 }
 
+license {
+    header = project.file('HEADER.txt')
+}
+
 sourceSets {
     ap {
         compileClasspath += main.compileClasspath + main.output