This change was directly committed into CraftBukkit short thereafter so this resource is now obsolete. Feel free to use it for learning purposes.
HeadFix is a resource designed to remove the new skull signature verification introduced into the Minecraft server in version 1.10+ This uses Java Agents to change the NBT packaging method's bytecode and remove the new verification. This means skull plugins such as this one by me and this one by sothatsit will still work in versions 1.10+
Either manually compile the project or download the jar from the releases. Paste the downloaded jar in your server's startup directory (the one that contains your spigot jar, your startup script, etc.) Next open your server's startup script in your favourite editor, it should look something like this:
java -jar spigot-1.10.2.jar
Edit this to include the javaagent parameter, it should look like this:
java -javaagent:"HeadFix-v1.0-SNAPSHOT.jar" -jar spigot-1.10.2.jar
Start the server and... that's it! You're done, the signature verification will be removed at runtime and your favourite head plugin will begin working again!