Updated readme to remove an unclaimed heroku host

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As part of addressing omniauth vulnerabilities, we vendored the omniauth-saleforce gem temporarily to support omniauth v2. The readme for the gem duplicates a URL to a Heroku instance hosting the demo app that GitLab does not own and should be removed in the vendored version. Resolves Expiry of herokuapp makes the official Gitlab d... (#393526 - closed)

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