Make greenhouse_io an optional dependency

[kbruccoleri]

Summary

• Source greenhouse_io from GitHub (grnhse/greenhouse_io) instead of RubyGems
• Remove greenhouse_io from the gemspec runtime dependencies so station no longer appears on greenhouse_io's reverse dependencies
• Make greenhouse_io an optional dependency: the app gracefully returns empty results when the gem is not present

Test plan

• Verify Greenhouse.careers, Greenhouse.devrel_careers, and Greenhouse.offices return data when greenhouse_io is available
• Verify the app boots and returns empty career listings when greenhouse_io is not installed

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