defacing option, upgraded dependencies

• added option for defacing (-D, disabled by default)
  • uses single T1w image (first alphanumeric match of _T1w) to deface with pydeface
  • applies it to any images in anat/ with the same FOV
  • backs-up images without defacing in sourcedata/no_defaced
  • TO DO: enable defacing of other filetypes if desired
• added "--overwrite" option as default for heudiconv, so heuristic doesn't fail when duplicates encountered
• Dockerfile now contains lines for setting heudiconv, bids-validator, and pydeface versions
  • currently: heudiconv=v0.5.3, validator=1.1.3, pydeface=v1.1.0
• added flirt and fslorient Dockerfile dependencies from dropbox (without entire FSL install) for pydeface