Refresh mkdocs dependencies

Author: mdmintz

mkdocs_build/requirements.txt

@@ -21,7 +21,7 @@ lunr==0.6.2
 nltk==3.8.1
 tornado==6.3.2
 watchdog==3.0.0
-cairocffi==1.5.1
+cairocffi==1.6.0
 cairosvg==2.7.0
 cssselect2==0.7.0
 tinycss2==1.2.1