Circumvent AttributeError when pushing tags

[elmatze79]

This is to fix this problem:

$ git tag -a example ae68de323acdc8181f9133c49749fd059ec7e23c
$ git push --tags

remote: An error occurred, but the commits are accepted.
remote: Traceback (most recent call last):
remote:   File "/usr/lib/python2.7/dist-packages/igcommit/prereceive.py", line 101, in main
remote:     state = Runner().run()
remote:   File "/usr/lib/python2.7/dist-packages/igcommit/prereceive.py", line 34, in run
remote:     check.print_problems()
remote:   File "/usr/lib/python2.7/dist-packages/igcommit/base_check.py", line 75, in print_problems
remote:     for severity, problem in self.get_problems():
remote:   File "/usr/lib/python2.7/dist-packages/igcommit/commit_list_checks.py", line 80, in get_problems
remote:     author_timestamp = commit.get_author().timestamp
remote:   File "/usr/lib/python2.7/dist-packages/igcommit/git.py", line 101, in get_author
remote:     return self._author
remote: AttributeError: 'Commit' object has no attribute '_author'

[hasegeli]

How about #12 instead?