src/telegram: Use $NTBA_FIX_319 to apply fix for #319

Notes:

  The permanent fix introduces backwards-incompatible changes, which
  require a major version bump. Until we release v1,
  environment variable `${NTBA_FIX_319}` can be used to apply
  the fix.

References:

  * BR: https://github.com/yagop/node-telegram-bot-api/issues/319

src/telegram.js

@@ -50,8 +50,9 @@ const _deprecatedMessageTypes = [
 ];
 
 
-// Enable Promise cancellation.
+if (!process.env.NTBA_FIX_319) {
-try {
+  // Enable Promise cancellation.
+  try {
     const msg =
       'Automatic enabling of cancellation of promises is deprecated.\n' +
       'In the future, you will have to enable it yourself.\n' +
@@ -60,7 +61,7 @@ try {
     Promise.config({
       cancellation: true,
     });
-} catch (ex) {
+  } catch (ex) {
     /* eslint-disable no-console */
     const msg =
       'error: Enabling Promise cancellation failed.\n' +
@@ -68,6 +69,7 @@ try {
     console.error(msg);
     throw ex;
     /* eslint-enable no-console */
+  }
 }