Updates nupic.core to df86d7f6c61296a2524a86524f7db16d6a4d60fe.
Merge pull request #979 from numenta-ci/core-update-df86d7f6c61296a2524a86524f7db16d6a4d60fe

Updates nupic.core to df86d7f6c61296a2524a86524f7db16d6a4d60fe.