gitlab: fix i686-linux build and module

It turns out that installing therubytracer, with dependency on old v8, even
when using source libv8 version is problematic.
(see
http://stackoverflow.com/questions/21666379/problems-installing-gitlab-on-odroid-v8-lib-not-available).

But wait, rails does not even need therubytracer, just any kind of javascript
server side execution framework like nodejs. Well just use that, as also
suggested from different internet sources (look link above), it works just
fine.
nixos/gitlab: imporove tests