yet another fix for travis
yet another fix for travis

Summary:
fix adapted from
https://github.com/travis-ci/travis-ci/issues/6120#issuecomment-22407254
0
Closes https://github.com/facebook/wdt/pull/124

Reviewed By: uddipta

Differential Revision: D3473924

Pulled By: ldemailly

fbshipit-source-id: 45fef572a145c9eba7937ea920dd4f17204eec97

luluandleilei/wdt,ldemailly/wdt,ldemailly/wdt,ldemailly/wdt,luluandleilei/wdt,luluandleilei/wdt,ldemailly/wdt,luluandleilei/wdt