provide a user agent when fetching the releases entry

src/SyncReleases/SyncImplementations.cs

@@ -99,7 +99,10 @@ namespace SyncReleases
         {
             Console.WriteLine("Trying to download RELEASES index from {0}", uri);
 
+            var userAgent = new System.Net.Http.Headers.ProductInfoHeaderValue("Squirrel", Assembly.GetExecutingAssembly().GetName().Version.ToString());
+
             using (HttpClient client = new HttpClient()) {
+                client.DefaultRequestHeaders.UserAgent.Add(userAgent);
                 return await client.GetStringAsync(uri);
             }
         }