Fix tests on Travis CI

Error was:

2013-02-27 14:36:11,509 - auto_version - INFO - executing command: git add testfile1.txt testfile2.txt

*** Please tell me who you are.

Run

  git config --global user.email "you@example.com"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

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ERROR: test_perform (tests.parsers_test.TestBasicParser)
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Traceback (most recent call last):
  File "/home/travis/build/paulollivier/auto_versionning/tests/parsers_test.py", line 48, in test_perform
    self.assertEqual("0.1", self.p.perform())
  File "/home/travis/build/paulollivier/auto_versionning/auto_version/parsers.py", line 115, in perform
    self.vcs.set_version(files=self.files, version=new_version, prefix=self.scm_prefix)
  File "/home/travis/build/paulollivier/auto_versionning/auto_version/dvcs.py", line 130, in set_version
    check_call(["git", "commit", "-m", "auto_version: added files for whom version has changed."])
  File "/usr/lib/python2.7/subprocess.py", line 511, in check_call
    raise CalledProcessError(retcode, cmd)
CalledProcessError: Command '['git', 'commit', '-m', 'auto_version: added files for whom version has changed.']' returned non-zero exit status 128