Automatically disconnect GoodData::Rest::Connection at Kernel#at_exit

See https://jira.intgdc.com/browse/GD-42506
Do not store Proc in @auth_handler, use @auth_handler_installed TrueClass/FalseClass
Fix rubocop issues
Merge remote-tracking branch 'upstream/master' into at-exit-disconnect
Mention at_exit handler in CHANGELOG.md
Merge branch 'master' of https://github.com/gooddata/gooddata-ruby into at-exit-disconnect
Merge branch 'master' of https://github.com/gooddata/gooddata-ruby into at-exit-disconnect

Conflicts:
	CHANGELOG.md
Merge pull request #372 from korczis/at-exit-disconnect

Automatically disconnect GoodData::Rest::Connection at Kernel#at_exit