include test/diff.txt in tarballs

[danc86]

This file is currently missing, which causes the tests to fail:

======================================================================
ERROR: test_diff_encoding (TestDiffEncoding.TestDiffEncoding)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "ansible-review-0.13.6/test/TestDiffEncoding.py", line 12, in test_diff_encoding
    with io.open(os.path.join(self.directory, 'diff.txt'), 'r') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'ansible-review-0.13.6/test/diff.txt'

[willthames]

Version 0.13.7 published with this change

[willthames]

Thanks @danc86