Fixed syntax for script step

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

.travis.yml

@@ -7,7 +7,7 @@ env:
   global:
   - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
 sudo: false
-script: jekyll build && htmlproofer ./_site
+script: bundle exec jekyll build && bundle exec htmlproofer ./_site
 before_deploy:
 - openssl aes-256-cbc -K $encrypted_518a90de15e0_key -iv $encrypted_518a90de15e0_iv
   -in deploy_ed25519.enc -out deploy_ed25519 -d