Remove references to Redis URL env variable

* This commit implements a change suggested by @lunks in https://github.com/thoughtbot/hound/issues/444
* To increase compatibility with redis-rb, remove references to `REDISTOGO_URL`
  environment variable in favor of `REDIS_URL`
* Reference: https://github.com/redis/redis-rb#getting-started
* 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