Revise setup instructions in README.md

* Set up local environment and clarified instructions in `README.md`
* Centralize on localhost:5000, since that's the default Foreman port
* Clearer GitHub and Stripe instructions

https://trello.com/c/TpFwhtlt/333-replicate-setup-locally-and-update-readme-me
Force UTF-8 encoding on file contents

`Base64.decode64` returns a string with encoding of `ASCII-8BIT`. Some linters
will try to write the file. If a file contains weird characters, an error will
get raised, like:

```
 Encoding::UndefinedConversionError: "\xE2" from ASCII-8BIT to UTF-8
```

Related error:
https://app.getsentry.com/app20126465herokucom/production/group/33706905/
Fix exemption check

* Check failed when more than one org was added. This change fixes that.
Configure Redis using redis-db defaults

* This commit implements a change suggested by @lunks in
  https://github.com/thoughtbot/hound/issues/444
* Remove Redis configuration in favor of redis-db's default configuration
* Reference: https://github.com/redis/redis-rb#getting-started
* Remove references to `REDISTOGO_URL` environment variable
* Note: Prior to merging and deploying this commit a `REDIS_URL`
  environment variable should be set on Heroku
* https://trello.com/c/AlmhdrFd/399-change-redistogo-url-environment-variable-to-redis-url