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

@@ -50,24 +50,26 @@ const _deprecatedMessageTypes = [
 ];
 
 
-// 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' +
-    'See https://github.com/yagop/node-telegram-bot-api/issues/319.';
-  deprecate(msg);
-  Promise.config({
-    cancellation: true,
-  });
-} catch (ex) {
-  /* eslint-disable no-console */
-  const msg =
-    'error: Enabling Promise cancellation failed.\n' +
-    '       Temporary fix is to load/require this library as early as possible before using any Promises.';
-  console.error(msg);
-  throw ex;
-  /* eslint-enable no-console */
+if (!process.env.NTBA_FIX_319) {
+  // 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' +
+      'See https://github.com/yagop/node-telegram-bot-api/issues/319.';
+    deprecate(msg);
+    Promise.config({
+      cancellation: true,
+    });
+  } catch (ex) {
+    /* eslint-disable no-console */
+    const msg =
+      'error: Enabling Promise cancellation failed.\n' +
+      '       Temporary fix is to load/require this library as early as possible before using any Promises.';
+    console.error(msg);
+    throw ex;
+    /* eslint-enable no-console */
+  }
 }