Use GitHub API to update humans.txt with 24pullrequests contributors

This PR is from the discussion in issue #658.

This updates the `human.txt` file via rake task in order to dynamically add the contributors to the list instead of manually adding them.

The snippet below is how I tested this since this can only be run in production

```ruby
# Before
Rails.application.config.contributors.map(&:login).map do |login|

#After
Octokit.configure do |c|
  c.access_token = ENV['GITHUB_API_TOKEN'] # This is already in my environment
end

Octokit.contributors('24pullrequests/24pullrequests').map(&:login).map do |login|
```

I'm open to suggestions on other implementations or advice on how to make this better.

Thanks!
I set it to be production only because it looks like Rails.application.config.contributers is only available in production.

I'm basing that theory on the snippet here https://github.com/24pullrequests/24pullrequests/blob/master/config/application.rb#L73