Fix conda update error in Travis

This should fix the error in Travis, according to
https://github.com/conda/conda/issues/1049#issuecomment-67176164

Original error:
https://travis-ci.org/CamDavidsonPilon/lifelines/jobs/44148107

Signed-off-by: Jonas Kalderstam <jonas@kalderstam.se>
Merge pull request #114 from CamDavidsonPilon/fix-travis

Fix conda update error in Travis