Relax nokogiri gemspec pinning

Dependencies conflicts with knife-ec2:

```
Bundler could not find compatible versions for gem "nokogiri":
  In Gemfile:
    foodcritic (= 3.0.3) ruby depends on
      nokogiri (~> 1.5.4) ruby

    knife-ec2 (= 0.8.0) ruby depends on
      nokogiri (1.6.1)
```