Add cache buster to all GitHub API calls (#449)

This should solve issues like #308 and the issues with cached
API responses after deleting an entry

src/backends/github/API.js

@@ -44,7 +44,8 @@ export default class API {
   }
 
   urlFor(path, options) {
-    const params = [];
+    const cacheBuster = new Date().getTime();
+    const params = [`ts=${cacheBuster}`];
     if (options.params) {
       for (const key in options.params) {
         params.push(`${ key }=${ encodeURIComponent(options.params[key]) }`);