GIT_API_URL in gecko_release_page_parser.rb should provide an option to use ACCESS_TOKEN to avoid rate limit

[hitliubo]

By default geckodriver-helper will check the latest release from github. However, there's a rate limit in GitHub which could be potentially blocked. In my case, in my company the default 60 per hour is not enough. So it would be nice if we could pass parameters such as ACCESS_TOKEN or something based on the GitHub documentation to avoid such rate limit.

https://developer.github.com/v3/#increasing-the-unauthenticated-rate-limit-for-oauth-applications
https://developer.github.com/v3/#oauth2-token-sent-in-a-header