Switch Markdown linter to node based markdownlint-cli.

I've prefered the node based linter to the ruby one, but it didn't have
a CLI until recently. Also the config file is in an easier format.

Author: waylan

.markdownlintrc

@@ -0,0 +1,16 @@
+{
+  // Enable all markdownlint rules
+  "default": true,
+
+  // Disable line length check for tables and code blocks
+  "MD013": { "line_length": 80, "code_blocks": false, "tables": false },
+
+  // Set Ordered list item prefix to "ordered" (use 1. 2. 3. not 1. 1. 1.)
+  "MD029": { "style": "ordered" },
+
+  // Set list indent level to 4 which Python-Markdown requires
+  "MD007": { "indent": 4 },
+
+  // Exclude code block style
+  "MD046": false
+}

.travis.yml

@@ -7,7 +7,7 @@ matrix:
     # Default Python
     - env: TOXENV=flake8
     - env: TOXENV=markdown-lint
-      before_install: gem install mdl
+      before_install: npm install -g markdownlint-cli
     - env: TOXENV=linkchecker
     - env: TOXENV=jshint
       before_install: npm install -g jshint

mdl_ruleset.rb

@@ -21,9 +21,9 @@ deps=-rrequirements/test.txt
 commands={envbindir}/flake8 mkdocs --max-line-length=119 --exclude=mkdocs/compat.py
 
 [testenv:markdown-lint]
-whitelist_externals = mdl
+whitelist_externals = markdownlint
 passenv = *
-commands=mdl --style mdl_ruleset.rb README.md CONTRIBUTING.md docs/
+commands=markdownlint README.md CONTRIBUTING.md docs/
 
 [testenv:linkchecker]
 basepython = python2.7