Disable RubyGems to increase performance

By default, RubyGems is enabled, which causes the Ruby interpreter to load 600+ additional files at startup when gitlab-shell is designed not to use any external gems.

For example, before:

# time /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell
Only ssh allowed

real    0m0.110s
user    0m0.094s
sys     0m0.017s

After:

# time /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell
Only ssh allowed

real    0m0.015s
user    0m0.012s
sys     0m0.004s