src/telegram: Fix download file path on windows

References:

    * BR: https://github.com/yagop/node-telegram-bot-api/issues/363
    * PR: https://github.com/yagop/node-telegram-bot-api/pull/364
    * PR-by: @kucherenkovova

src/telegram.js

@@ -1050,7 +1050,7 @@ class TelegramBot extends EventEmitter {
       .then(fileURI => {
         const fileName = fileURI.slice(fileURI.lastIndexOf('/') + 1);
         // TODO: Ensure fileName doesn't contains slashes
-        const filePath = `${downloadDir}/${fileName}`;
+        const filePath = path.join(downloadDir, fileName);
 
         // properly handles errors and closes all streams
         return Promise