Use m2r2 for now to fix build

Until Sphinx upstream moves to a different markdown converter,
m2r2 would fix builds.
Issue:
sphinx-doc/sphinx#7420
See also:
miyakogi/m2r#55 (comment)

.travis.yml

@@ -3,7 +3,7 @@ python: 3.7
 install:
 - pip install Sphinx
 - pip install git+https://github.com/bashtage/sphinx-material
-- pip install m2r
+- pip install m2r2
 script: skip
 deploy:
 - provider: script

conf.py

@@ -29,7 +29,7 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['m2r', 'sphinx_material']
+extensions = ['m2r2', 'sphinx_material']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']