Use the latest mysterious AMO signing API change

gulpfile.js

@@ -9,7 +9,7 @@ var merge = require("merge-stream");
 var del = require("del");
 var replace = require("gulp-replace");
 var gulpIf = require("gulp-if");
-var signAddon = require("sign-addon").default;
+var signAddon = require("sign-addon");
 var rollup = require("rollup");
 var resolve = require("@rollup/plugin-node-resolve").nodeResolve;
 var typescript = require("rollup-plugin-typescript2");