Codechange: Exclude vendor from build

vendor is the most likely place for someone to have put local gems.
If this is the case, jekyll will try to compile its own site template,
which is located in something like
vendor/cache/gems/jekyll-3.2.1/lib/site_template/_posts/0000-00-00-welcome-to-jekyll.markdown.erb,
which will then fail due to an invalid strftime.

See jekyll/jekyll#5267 for further details.

_config.yml

@@ -23,6 +23,7 @@ exclude:
   - nginx.default.conf
   - requirements.base
   - requirements.txt
+  - vendor
 
 defaults:
   - scope: