[VAGRANT] Fix incorrect Chef installation for CentOS

The script was also doing `yum update`, not really neccessary :), copy&paste fail

Vagrantfile

@@ -191,7 +191,7 @@ Vagrant::Config.run do |config|
         version = ENV['CHEF'].match(/^\d+/) ? ENV['CHEF'] : nil
         shell.inline = %Q{
           which apt-get > /dev/null 2>&1 && apt-get install curl --quiet --yes
-          which yum > /dev/null 2>&1 && yum update -y && yum install curl -y
+          which yum > /dev/null 2>&1 && yum install curl -y
           test -d "/opt/chef" || curl -# -L http://www.opscode.com/chef/install.sh | sudo bash -s -- #{version ? "-v #{version}" : ''}
           /opt/chef/embedded/bin/gem list pry | grep pry || /opt/chef/embedded/bin/gem install pry --no-ri --no-rdoc
         }