Update package-lock.json to latest version.

We should review whether or not to include this file in the source tree,
given that we use Yarn primarily for package management, but since it is
here, keep it up to date.

package-lock.json

 {
   "name": "hypothesis",
-  "version": "1.27.0",
+  "version": "1.28.0",
   "lockfileVersion": 1,
   "dependencies": {
     "@gulp-sourcemaps/map-sources": {