gitlab shell works if multiple rubies installed

Before this it would fail because git hooks automatically prepend
things to the path, which can lead the wrong Ruby version to be called
in which dependencies are not installed.

To make sure that this is correct, the forked_merge_requests commented
out test that depends on this change was uncommented.

For that test to pass, it is also necessary to setup the mock server
on port 3001 under test_env.rb.