Update plugin to export anonymously

This updates the plugin to anonymously export the Model extension as
suggested by @koskimas and accordingly with the update on the
Objection.js plugin best practices.

Check Vincit/objection.js#475 for more on best
practices update.

Closes #3.

index.js

@@ -18,7 +18,7 @@ module.exports = options => {
   }, options);
 
   return Model => {
-    return class UniquePlugin extends Model {
+    return class extends Model {
 
       /**
        * Before insert.