Potential CI issue in macOS

[Fullstop000]

https://travis-ci.org/Fullstop000/wickdb/jobs/588278442
CI failed in macOS because of the lack of lib six for lldb:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Library/Developer/CommandLineTools/Library/PrivateFrameworks/LLDB.framework/Resources/Python/lldb/__init__.py", line 98, in <module>
    import six
ImportError: No module named six
Traceback (most recent call last):
  File "<string>", line 1, in <module>

And it's wired that sstable::tests::test_simple_empty_key hangs when running kcov but works fine in other places.

Relative issue: rust-lang/rust#38380

[Fullstop000]

Fixed by 2d5e755