Respect custom gitlab_shell path in gitlab:gitlab_shell:check task

Prior to this change, if a user customized the config value of
`gitlab_shell.path`, the check would ignore this value in favor of
`"~<gitlab_shell.ssh_user>/gitlab-shell/"` resulting in a failed check
when the configuration was otherwise valid.

Why?

I develop on OSX, and after a recent reformat I found it way more difficult to
get a development environment for Gitlab running than it should have been. I
finally found a workable solution that involved no virtual machines and no
additional dedicated user, but this minor change was necessary to facilitate
it. I'll be writing a blog post a bit later describing what I finally got set
up.

Unless this seriously breaks something else, a merge would be appreciated.

Thanks!

Before:

```
❯ be rake gitlab:gitlab_shell:check
Digest::Digest is deprecated; use Digest
Checking GitLab Shell ...

GitLab Shell version >= 1.7.9 ? ... OK (1.8.0)
Repo base directory exists? ... yes
Repo base directory is a symlink? ... no
Repo base owned by tsigo:tsigo? ... no
  User id for tsigo: 501. Groupd id for tsigo: group tsigo doesn't exist
  Try fixing it:
  sudo chown -R tsigo:tsigo tmp/repositories/
  For more information see:
  doc/install/installation.md in section "GitLab Shell"
  Please fix the error above and rerun the checks.
Repo base access is drwxrws---? ... yes
update hook up-to-date? ... yes
update hooks in repos are links: ...
Documentcloud / Underscore ... ok
Diaspora / Diaspora ... ok
Diaspora / Diaspora Project Site ... ok
Diaspora / Diaspora Client ... ok
Brightbox / Brightbox Cli ... ok
Brightbox / Puppet ... ok
Gitlabhq / Gitlabhq ... ok
Gitlabhq / Gitlab Ci ... ok
Gitlabhq / Gitlab Recipes ... ok
Gitlabhq / Gitlab Shell ... ok
Gitlabhq / Grack ... ok
Gitlabhq / Testme ... ok
Twitter / Flight ... ok
Twitter / Typeahead.Js ... ok
H5bp / Html5 Boilerplate ... ok
H5bp / Mobile Boilerplate ... ok
Administrator / gitlab-self ... repository is empty
Running /Users/tsigo/gitlab-shell/bin/check
gitlab-shell self-check failed
  Try fixing it:
  Make sure GitLab is running;
  Check the gitlab-shell configuration file:
  sudo -u tsigo -H editor /Users/tsigo/gitlab-shell/config.yml
  Please fix the error above and rerun the checks.

Checking GitLab Shell ... Finished
```

After:

```
❯ be rake gitlab:gitlab_shell:check
Digest::Digest is deprecated; use Digest
Checking GitLab Shell ...

GitLab Shell version >= 1.7.9 ? ... OK (1.8.0)
Repo base directory exists? ... yes
Repo base directory is a symlink? ... no
Repo base owned by tsigo:tsigo? ... no
  User id for tsigo: 501. Groupd id for tsigo: group tsigo doesn't exist
  Try fixing it:
  sudo chown -R tsigo:tsigo tmp/repositories/
  For more information see:
  doc/install/installation.md in section "GitLab Shell"
  Please fix the error above and rerun the checks.
Repo base access is drwxrws---? ... yes
update hook up-to-date? ... yes
update hooks in repos are links: ...
Documentcloud / Underscore ... ok
Diaspora / Diaspora ... ok
Diaspora / Diaspora Project Site ... ok
Diaspora / Diaspora Client ... ok
Brightbox / Brightbox Cli ... ok
Brightbox / Puppet ... ok
Gitlabhq / Gitlabhq ... ok
Gitlabhq / Gitlab Ci ... ok
Gitlabhq / Gitlab Recipes ... ok
Gitlabhq / Gitlab Shell ... ok
Gitlabhq / Grack ... ok
Gitlabhq / Testme ... ok
Twitter / Flight ... ok
Twitter / Typeahead.Js ... ok
H5bp / Html5 Boilerplate ... ok
H5bp / Mobile Boilerplate ... ok
Administrator / gitlab-self ... repository is empty
Running /Users/tsigo/Code/external/gitlab-shell/bin/check
Check GitLab API access: OK
Check directories and files:
	/Users/tsigo/Code/rails/gitlabhq/tmp/repositories/: OK
	/Users/tsigo/Code/external/gitlab-shell/authorized_keys: OK
Test redis-cli executable: redis-cli 2.8.7
Send ping to redis server: PONG
gitlab-shell self-check successful

Checking GitLab Shell ... Finished
```