Add HEROKU_REGION to configure app region

[jamesjoshuahill]

Thanks for contributing to postfacto. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:

  This change enables you to deploy Postfacto to a Heroku region of your choice.
  Run heroku regions to list available regions.

  Given I have set $HEROKU_REGION
  When I run the Heroku deployment script
  Then the app will be deployed into the configured region

  Example:

  HEROKU_REGION='eu' ./deploy.sh <app-name>

  When HEROKU_REGION is not set the app will deploy to the default us region.

• An explanation of the use cases your change solves

  Allows deployment to the Heroku region nearest to your Postfacto users to reduce latency.

• Links to any other associated PRs

  None.

• I have reviewed the contributing guide

• I have made this pull request to the master branch

• I have run all the tests using ./test.sh.

• I have added the copyright headers to each new file added

• I have given myself credit in the humans.txt file (assuming I want to)

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[pivotal-issuemaster]

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[jamesjoshuahill]

Thank you @textbook 🙏🏻