Specify mkdocs version for pip

Newest mkdocs (1.7.0) conflicts.  Specifying 1.6.8.

Issue-Id: DCAEGEN2-128
Change-Id: I4e6c5dc52db41da1908fee75cfd219f89534fb2c
Signed-off-by: Lusheng Ji <lji@research.att.com>
1 file changed