Use httpcache library for GH response caching

[MatousJobanek]

The go-github library doesn't handle GH conditional requests directly but provides a possibility to work with a caching on the level of http.Transport. https://github.com/google/go-github/#conditional-requests
They recommend using https://github.com/gregjones/httpcache library for that. Since it should work as RFC 7234 compliant cache for http responses then it should minimize the GH request rate limit usage - it's using the same headers as GH conditional requests

[MatousJobanek]

I'm moving this issue to icebox as (according to the metrics) it seems that we don't use many of the requests. In other words, it doesn't seem to be very important to spend time with this issue and to add an additional logic to the project when there is no danger that we could run out of the limited number of requests.