Support REDIS_PROVIDER variable used in Heroku

[fluke]

Heroku Redis services often use the REDIS_PROVIDER variable to set the name of the variable which contains the Redis URL I'm using config.redis_url option but I think we should add support for REDIS_PROVIDER so Split works with minimal configuration.

As you can see Sidekiq also supports it:
https://github.com/mperham/sidekiq/blob/master/lib/sidekiq/redis_connection.rb#L93

def determine_redis_provider
  ENV[ENV['REDIS_PROVIDER'] || 'REDIS_URL']
end

In this pull request I've followed that method but used ENV.fetch as that was what was used before.

[coveralls]

Coverage remained the same at 97.952% when pulling 911edd0 on kartikluke:master into 4f777c1 on splitrb:master.

[andrew]

Nice one, will get a new release with that included out later today 👌

[andrew]

Published as 2.1.0: https://github.com/splitrb/split/releases/tag/v2.1.0 🚢