Refactor to use current (3.x) async package

[waltjones]

Description of the change

The async package dependency needed to be updated from v1.x to 3.x to pick up a security update. There have been interface changes since then, so a small refactor was also needed to work with the current async interface.

The specific changes that affect rollbar.js happened in async v2.0.0:

filter, reject, some, every, detect and their families like {METHOD}Series and {METHOD}Limit now expect an error as the first callback argument, rather than just a simple boolean.

https://github.com/caolan/async/blob/master/CHANGELOG.md#breaking-changes-1

This PR updates the calling convention for callbacks, and removes usage of fs.exists which is is both deprecated and incompatible with the current async interface.

Type of change

• Bug fix (non-breaking change that fixes an issue)

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[cyrusradfar]

Thanks for the background on the changes in the new version of async.