Bump nokogiri dependency to the latest stable version

[joelhandwell]

ChefDK 0.18.26 released with Berkshelf 5.1.0 and embedded Ruby 2.3.1. To use Ruby 2.3.1 and berkshelf 5.1.0 together with vagrant-berkshelf plugin, vagrant-berkshelf plugin have to update nokogiri dependency to 1.6.8 because nokogiri 1.6.7.1 is not compatible with Ruby < 2.3 as following:

C:\oss\vagrant-berkshelf>gem install nokogiri -v '1.6.7.1'
WARNING:  You don't have c:\users\joel\appdata\local\chefdk\gem\ruby\2.3.0\bin in your PATH,
          gem executables will not run.
ERROR:  Error installing nokogiri:
        nokogiri requires Ruby version < 2.3, >= 1.9.2.

And when nokogiri 1.6.8 is set as dependency in vagrant-berkshelf Gemfile, following error is shown:

C:\oss\vagrant-berkshelf>bundle install
Your Gemfile lists the gem vagrant-berkshelf (>= 0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
Fetching git://github.com/mitchellh/vagrant.git
Fetching gem metadata from https://rubygems.org/
Fetching version metadata from https://rubygems.org/
Fetching dependency metadata from https://rubygems.org/
Resolving dependencies..........
Bundler could not find compatible versions for gem "nokogiri":
  In Gemfile:
    nokogiri (>= 1.6.8) x86-mingw32

    vagrant x86-mingw32 was resolved to 1.8.6.dev, which depends on
      nokogiri (= 1.6.7.1) x86-mingw32

Above error suggests nokogiri dependency needs to be upgraded in vagrant first, then vagrant-berkshelf can upgrade nokogiri dependency to 1.6.8, then ChefDK 0.18.26 can be used with vagrant-berkshelf plugin without following error:

C:\Users\Joel\test>vagrant up test
Bringing machine 'test' up with 'virtualbox' provider...
The Berkshelf version at "C:\\opscode\\chefdk\\embedded\\bin/berks.BAT" is invalid.
Vagrant Berkshelf requires ~> 4.0, but the current version is 5.1.0.

Please download and install the latest version of the ChefDK from:

    https://downloads.chef.io/chef-dk

and follow the installation instructions. Do not forget to add the ChefDK to
your PATH.

The environment of this context is Windows 10.

#6766, berkshelf/vagrant-berkshelf#305, berkshelf/vagrant-berkshelf#306 is related with this.

[littlesnitch]

+1
Is important for us as well. We would appreciate this commit.

[joelhandwell]

Closing this for #8571 covers

[ghost]

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