License finder gem

Every time a gem is added to the Gemfile, or a gem is updated with a new dependency or change of license, LicenseFinder will check to ensure that the license in use has been whitelisted for use in the project. GPLv2 and GPLv3 libraries are not allowed to be linked-to from non-GPL projects (e.g. the MIT-licensed GitLab CE or proprietary EE), otherwise we're violating the license.

https://github.com/pivotal/LicenseFinder

See also: gitlab-com/operations#164

See merge request !3775