Remove use of deprecated functions from test code

lib/constructor.js

@@ -14,6 +14,7 @@ let versions = {
 (function () {
   // Does libvips meet minimum requirement?
   const libvipsVersionMin = require('../package.json').config.libvips;
+  /* istanbul ignore if */
   if (semver.lt(versions.vips, libvipsVersionMin)) {
     throw new Error('Found libvips ' + versions.vips + ' but require at least ' + libvipsVersionMin);
   }