Fix markdownlint test (#2095)

Travis only gives us Node.js version 8 so we must use an older
version of markdownlint. To declare a newer node.js version we
lose the ability to have Python as the lang and install tox.
If/when the default node version gets updated, remove the
version restriction on markdownlint.

.travis.yml

@@ -7,7 +7,12 @@ matrix:
     # Default Python
     - env: TOXENV=flake8
     - env: TOXENV=markdown-lint
-      before_install: npm install -g markdownlint-cli
+      # Travis only gives us Node.js version 8 so we must use an older
+      # version of markdownlint. To declare a newer node.js version we
+      # lose the ability to have Python as the lang and install tox.
+      # If/when the default node version gets updated, remove the
+      # version restriction on markdownlint.
+      before_install: npm install -g markdownlint-cli@0.21.0
     # Until Linkchecker is updated to Python 3, we will have skip this test.
     # Watch https://github.com/linkchecker/linkchecker for updates.
     # - env: TOXENV=linkchecker

docs/user-guide/custom-themes.md

@@ -60,7 +60,7 @@ theme:
 
 !!! Warning
 
-    A theme's [configuration] defined in a `mkdocs_theme.yml` file is not loaded 
+    A theme's [configuration] defined in a `mkdocs_theme.yml` file is not loaded
     from `theme.custom_dir`. When an entire theme exists in `theme.custom_dir`
     and `theme.name` is set to `null`, then the entire theme configuration must
     be defined in the [theme] configuration option in the `mkdocs.yml` file.

tox.ini

@@ -20,9 +20,14 @@ deps=-rrequirements/test.txt
 commands={envbindir}/flake8 mkdocs --max-line-length=119
 
 [testenv:markdown-lint]
-whitelist_externals = markdownlint
+whitelist_externals=
+    node
+    markdownlint
 passenv = *
-commands=markdownlint README.md CONTRIBUTING.md docs/
+commands=
+    node --version
+    markdownlint --version
+    markdownlint README.md CONTRIBUTING.md docs/ --ignore docs/CNAME
 
 [testenv:linkchecker]
 basepython = python2.7