Update Mkdocs Requirements

This patch updates `mkdocs.txt`, the Mkdocs requirements file, to
specify version >=0.16.3,<0.17.0 for mkdocs as 0.17.0 breaks RTFD per
readthedocs/readthedocs.org#3174.

mkdocs.txt

@@ -1,2 +1,2 @@
-mkdocs >= 0.16.3
+mkdocs >= 0.16, < 0.17
 mkdocs-bootswatch >= 0.4.0