Made sure the dependencies are cached

Signed-off-by: Jim Martens <github@2martens.de>

.travis.yml

@@ -7,6 +7,7 @@ env:
   global:
   - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
 sudo: false
+cache: bundler
 script: bundle exec jekyll build && bundle exec htmlproofer ./_site --disable-external
 before_deploy:
 - openssl aes-256-cbc -K $encrypted_518a90de15e0_key -iv $encrypted_518a90de15e0_iv