Add nokogiri -v to .travis.yml

Author: junaruga

.travis.yml

@@ -2,6 +2,8 @@ language: ruby
 sudo: false
 cache: bundler
 before_install: gem update bundler --no-document
+before_script:
+  - bundle exec nokogiri -v
 rvm:
   - 1.9.3
   - 2.0.0