Fix URL concatenation

Some servers (*cough*Cloudfront*cough*) reject double-slashes, so blind
string concatenation (nodejs/node-v0.x-archive#2216 (comment))
is not adequate to combine URLs.

package.js

@@ -15,6 +15,7 @@ Npm.depends({
   "ncp": "2.0.0",
   "rimraf": "2.4.4",
   "semver": "5.1.0",
+  "url-join": "0.0.1"
 });
 
 Package.on_use(function (api) {

server/downloadUrls.js

@@ -1,4 +1,4 @@
-var path = Npm.require('path');
+var urlJoin = Npm.require('url-join');
 
 DOWNLOAD_URLS = {};
 
@@ -7,7 +7,7 @@ var latestVersion = electronSettings.version;
 if (electronSettings.downloadUrls) {
   if (electronSettings.downloadUrls.win32) {
     // 'AppSetup.exe' refers to the output of `grunt-electron-installer`.
-    DOWNLOAD_URLS['win32'] = path.join(electronSettings.downloadUrls.win32, electronSettings.name + 'Setup.exe');
+    DOWNLOAD_URLS['win32'] = urlJoin(electronSettings.downloadUrls.win32, electronSettings.name + 'Setup.exe');
   }
   if (electronSettings.downloadUrls.darwin){
     DOWNLOAD_URLS['darwin'] = electronSettings.downloadUrls.darwin.replace('{{version}}', latestVersion);

server/serveUpdateFeed.js

@@ -1,5 +1,6 @@
 var path = Npm.require('path');
 var semver = Npm.require('semver');
+var urlJoin = Npm.require('url-join');
 var electronSettings = Meteor.settings.electron || {};
 var latestVersion = electronSettings.version;
 
@@ -42,8 +43,7 @@ serveUpdateFeed = function() {
       //first strip off the UPDATE_FEED_PATH
       var path = req.url.split(UPDATE_FEED_PATH)[1];
       res.statusCode = 302;
-      // Can't use `path.join` since this is a URL. Two slashes in a row are fine if present.
-      res.setHeader("Location", windowsDownloadPrefix + '/' + path);
+      res.setHeader("Location", urlJoin(windowsDownloadPrefix, path));
       res.end();
     });
   }