Exclude node_modules directory from Jekyll build

The primer modules in use all have README.md files
which are publicly accessible.

This PR removes them from the Jekyll build.

Author: MrChrisW

_config.yml

@@ -7,7 +7,7 @@ relative_permalinks: false
 analytics_account: "UA-3769691-28"
 
 # build settings
-exclude: ["script", "vendor", "bower.json", "Gemfile", "Gemfile.lock", "Rakefile", "readme.md", "package.json"]
+exclude: ["script", "vendor", "bower.json", "Gemfile", "Gemfile.lock", "Rakefile", "readme.md", "package.json", "node_modules"]
 markdown: kramdown
 plugins:
   - jekyll-avatar