.travis.yml: initial html5validation

Currently it's forced to be true, because it fails too much due to the
validator being old and not understanding Upgrade Insecure Requests.

.travis.yml

@@ -1,6 +1,11 @@
 language: ruby
 sudo: false
-script: "set -e && gem install github-pages && jekyll build --drafts"
+script:
+  - "gem install github-pages"
+  - "jekyll build --drafts"
+    "python2 --version && python2 -m pip --version"
+  - "python2 -m pip install html5validator --user --upgrade"
+  - "~/.local/bin/html5validator --root _site/|true"
 rvm: 2.1
 notifications:
     email: false