update Travis config

.travis.yml

@@ -1,4 +1,17 @@
-install: gem install github-pages
-script: jekyll build --trace
 sudo: false
-rvm: 2.1
+language: ruby
+cache: bundler
+rvm: 2.4
+
+before_install:
+  - gem update --system
+install: bundle install
+script: bundle exec jekyll build --trace
+
+branches:
+  only:
+  - gh-pages
+
+env:
+  global:
+  - NOKOGIRI_USE_SYSTEM_LIBRARIES=true