propably better ssh-key id finding in ARGV

I have issue with fresh instalation of gitlab on debian squeze

gitlab-shell cannot verify my ssh-key on issuing git commands

variable ARGV have inside some random parameters.
I dont find from where this '-c' come.

this ends with wrong call to gitlab api
```
/api/v3/internal/allowed?key_id=-c&action=git-upload-pack&ref=_any&project=user/repo
```

so i replace simple ARGV.shift with regex

content of ARGV on my installation
```
-c
/home/git/gitlab-shell/bin/gitlab-shell key-4
```