fix: remove dependency on when in favor of native Promises

when is a peculiar library that cannot be bundled by tools like esbuild.
Additionally, Promise implementations are available in all currently supported
Node releases, so we can remove this dependency whithout impacting user ability
to override `promiseFactory`.

index.js

@@ -8,7 +8,6 @@
  *
  */
 var extend = require('extend');
-var when = require('when');
 var request = require('request');
 var RetryStrategies = require('./strategies');
 var _ = require('lodash');
@@ -22,7 +21,7 @@ var DEFAULTS = {
 
 // Default promise factory which use bluebird
 function defaultPromiseFactory(resolver) {
-  return when.promise(resolver);
+  return new Promise(resolver);
 }
 
 function _cloneOptions(options) {

package.json

@@ -44,8 +44,7 @@
   },
   "dependencies": {
     "extend": "^3.0.2",
-    "lodash": "^4.17.15",
-    "when": "^3.7.7"
+    "lodash": "^4.17.15"
   },
   "peerDependencies": {
     "request": "2.*.*"