Use https protocol instead of git in Gemfile.

Due to https://shellycloud.com/documentation/faq#gems_over_ssh
Removed shelly gem from Gemfile.

It is recommended to remove shelly gem from Gemfile
https://github.com/shellycloud/shelly/blob/master/lib/shelly/cli/main/check.rb#L38-L42
Added poltergeist for integration tests.