Ensure GithubImport::Client does not crash when the 'verify_ssl' config is not present

This fixes gitlab-org/gitlab-ce#14343 and !260 (merged) when verify_ssl is not present in the GitHub omniauth settings in gitlab.yml.

Settingslogic allow to access optional settings through the Hash notation: if the key is not present, it returns nil. In the present case this will let the Octokit::Client object use whatever default value it wants for the ssl option.

/cc @dbalexandre