added activity photos back

[simonneutert]

Merging this PR

• finishes what was started in https://github.com/dblock/strava-ruby-client/pull/83 by @dillon-co, thank you very much and congrats to your first contribution here 🙏
• fixes the specs by refreshing the cassette and checking manually the handed out URL from strava's cdn
• adds changelog entries
• adds a sample comment in the README on how the size parameter can be utilised to get photos in a certain size

misc

I had to add faraday-retry as a dependency in order to get Danger to work properly with the CI.