fix: Use bugsnag/tap for Homebrew installation

[xljones]

Goal

Move Homebrew installation to Bugsnag tap, and fix missing SHA256 checksum error thrown when installing directly from formula.

Note: This PR is only to fix documentation and remove the old formula. The new tap, and formula can be found here: https://github.com/bugsnag/homebrew-bugsnag-tap

Fixes #46 Fixes #16

Design

This approach allows users to install the bugsnag-dsym-upload tool without error. Additionally, it allows users to keep up to date with changes to this tool through brew directly, rather than needing to explicitly update this single formula.

Changeset

• Removed /tools/homebrew/bugsnag-dsym-upload.rb
• Updated contributing & readme files to reflect the changes.
• Added new repo https://github.com/bugsnag/homebrew-bugsnag-tap with tap & formula for this tool.

Testing

• Added brew workflow to the new tap & formula repo: https://github.com/bugsnag/homebrew-bugsnag-tap
• Tested new install method manually.