Replaces fixer.io with frankfurter.app

Fixer.io has been converted payed subscription-based solution. Therefore, I have changed to a free alternative.

Author: KristofferRisa

src/Wox.Plugin.Currency/CurrencyPlugin.cs

@@ -103,7 +103,7 @@ private List<Result> LoadCurrency(List<Result> results)
             {
                 Title = $"{_money.ToString(".00")} {_fromCurrency} = {(_money*rate).ToString("C")} {_toCurrency}",
                 IcoPath = "Images/bank.png",
-                SubTitle = $"Source: fixer.io (Last updated {currency.date})"
+                SubTitle = $"Source: https://frankfurter.app (Last updated {currency.date})"
             });
             return results;
         }
@@ -116,7 +116,7 @@ public void Init(PluginInitContext context)
         #region helpers
         private Models.Currency GetCurrency(SearchParameters searchParameters)
         {
-            var url = $"http://api.fixer.io/latest?base={searchParameters.BaseIso}&symbols={searchParameters.ToIso}";
+            var url = $"https://frankfurter.app/latest?base={searchParameters.BaseIso}&symbols={searchParameters.ToIso}";
 
             if (_cache.ContainsKey(searchParameters))
             {