Update mkdocs version to 0.12.2

The old version of mkdocs required 'Markdown>=2.3.1,<2.5' and
requirements-optionals.txt lists markdown==2.5.2 causing mkdocs
to fail.

requirements/requirements-documentation.txt

 # MkDocs to build our documentation.
-mkdocs==0.11.1
+mkdocs==0.12.2