Update git submodules

* Update validations-common from branch 'master'
  to 956de6764f9619e96090dbc7255f743f406f4970
  - Correct the tox option for skipping sdist generation
    
    The tox option to skip source distribution building is skipsdist,
    but this seems to be often misspelled skipdist instead, which gets
    silently ignored and so does not take effect. Correct it
    everywhere, in hopes that new projects will finally stop copying
    this mistake around.
    
    See https://tox.readthedocs.io/en/latest/config.html#conf-skipsdist
    and tox-dev/tox#1388 for details.
    
    Change-Id: Icad8c8c48f7b34002a60aecc6b82e6ac56fd8a70