Update installation instructions

[StephenHulme]

Update installation instructions

Changes proposed in this pull request

• Replace references to install from RubyGems with GitHub source tags

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.13%. Comparing base (ab93774) to head (9ff8bbb).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #62   +/-   ##
=======================================
  Coverage   65.13%   65.13%           
=======================================
  Files          13       13           
  Lines         218      218           
  Branches       21       21           
=======================================
  Hits          142      142           
  Misses         76       76           

Flag	Coverage Δ
pull_request	65.13% <ø> (?)
push	65.13% <ø> (ø)

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