GitLab /api/allowed endpoint requires POST request

This commit made changes to GitLab shell to work with huge pushed (ex.
1k branhes) using POST request to API

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'post-api' into 'master'

Make gitlab-shell work with new /api/allowed method

See merge request !39