Remove arrow functions in browser target

[waltjones]

Description of the change

This PR removes arrow functions that aren't compatible with IE11 and some other browsers. (The Rollbar.js build process currently doesn't include a transpile step that would make this compatible.)

Type of change

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

Related issues

Fixes https://github.com/rollbar/rollbar.js/issues/1064

https://app.shortcut.com/rollbar/story/120517

Development

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

Code review

• [x] 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
• [x] Issue from task tracker has a link to this pull request
• [ ] Changes have been reviewed by at least one other engineer