Rename the gem

Related to gitlab-org/gitlab#395623 (closed)

The gem should be named the same as the file that we want auto-loaded (in our case: this one). Otherwise, applications that include this gem would need to have an additional require xxxx clause in their code (in this case - it would be require gitlab_sdk). This makes it so that we use a hypen-notation instead of usual snake_case, but it's an approach used in other gems, for example here.