Use nokogiri on Travis-ci

### 症状
Gemfileに ``` gem 'nokogiri' ``` 
と追加するとerrorが以下のように出た
```
You are trying to install in deployment mode after changing
your Gemfile. Run `bundle install` elsewhere and add the
updated Gemfile.lock to version control.
You have added to the Gemfile:
* nokogiri
```

#解決策
.travis.yml に
```
before_install:
  - gem install nokogiri -- --with-cflags='--std=gnu99'
```
を記述すれば取り敢えず通る