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>
diff --git a/mvn-phase-script.sh b/mvn-phase-script.sh
index 7a2acb6..6bc987f 100755
--- a/mvn-phase-script.sh
+++ b/mvn-phase-script.sh
@@ -76,7 +76,7 @@
     virtualenv ./venv-doc
     source ./venv-doc/bin/activate
     pip install --upgrade pip
-    pip install --upgrade mkdocs mkdocs-material
+    pip install --upgrade 'mkdocs==0.16.3' mkdocs-material
     pip freeze
 
     mkdocs build