Use safe get for large number of builds

For large project (mine is 166GB of build artifacts), the use of listing all builds can fail with the following message:

# time ./gitlab-artifact-cleanup --project mygroup/myproject --min-age "6 months" --gitlab gitlab --dry-run                                                                                                                                                                           
Traceback (most recent call last):
Would delete non-tagged artifacts older than 180 days, 0:00:00

mygroup / myproject
  File "./gitlab-artifact-cleanup", line 214, in <module>
    main()
  File "./gitlab-artifact-cleanup", line 206, in main
    cleanup.cleanup_project(proj)
  File "./gitlab-artifact-cleanup", line 45, in cleanup_project
    for build in proj.builds.list(all=True):
  File "/tmp/gitlab-artifact-cleanup/local/lib/python2.7/site-packages/gitlab/objects.py", line 122, in list
    return self.obj_cls.list(self.gitlab, **args)
  File "/tmp/gitlab-artifact-cleanup/local/lib/python2.7/site-packages/gitlab/objects.py", line 258, in list
    return gl.list(cls, **kwargs)
  File "/tmp/gitlab-artifact-cleanup/local/lib/python2.7/site-packages/gitlab/__init__.py", line 407, in list
    return self._raw_list(url, obj_class, **kwargs)
  File "/tmp/gitlab-artifact-cleanup/local/lib/python2.7/site-packages/gitlab/__init__.py", line 351, in _raw_list
    raise e
gitlab.exceptions.GitlabConnectionError: Can't connect to GitLab server (maximum recursion depth exceeded in cmp)

real    0m40.867s
user    0m1.160s
sys     0m0.052s

I didn't need to do this for a project which is 27GB for example.

The documentation of python-gitlab (http://python-gitlab.readthedocs.io/en/stable/api-usage.html#pagination) mention this case by using safe_all instead of all.