Use appropriate function call for chef 12

In chef12 the function locate_config_file in Chef::Knife was renamed
to chef_config_dir.  This patch adds a conditional to check for chef12
and apply the appropriate function call.

.travis.yml

@@ -4,4 +4,3 @@ rvm:
   - 2.1.1
 
 script: bundle exec rake spec
-

lib/cheffish.rb

@@ -36,7 +36,11 @@ def self.profiled_config(config = Chef::Config)
   end
 
   def self.load_chef_config(chef_config = Chef::Config)
-    chef_config.config_file = Chef::Knife.locate_config_file
+    if ::Gem::Version.new(::Chef::VERSION) >= ::Gem::Version.new('12.0.0')
+      chef_config.config_file = ::Chef::Knife.chef_config_dir
+    else
+      chef_config.config_file = ::Chef::Knife.locate_config_file
+    end
     config_fetcher = Chef::ConfigFetcher.new(chef_config.config_file, chef_config.config_file_jail)
     if chef_config.config_file.nil?
       Chef::Log.warn("No config file found or specified on command line, using command line options.")