Add deprecation warning for WickedPdf.config=

This adds a deprecation warning for the old way to set config. Users
have the option to silence the warning if they want to do so.

lib/wicked_pdf.rb

@@ -20,10 +20,18 @@
 class WickedPdf
   DEFAULT_BINARY_VERSION = Gem::Version.new('0.9.9')
   @@config = {}
-  cattr_accessor :config
+  cattr_accessor :config, :silence_deprecations
 
   include Progress
 
+  def self.config=(config)
+    unless @@silence_deprecations
+      ::Kernel.warn "WickedPdf.config= is deprecated and will be removed in future versions. Use WickedPdf.configure instead."
+    end
+
+    @@config = config
+  end
+
   def self.configure
     config = OpenStruct.new(@@config)
     yield config

test/test_helper.rb

@@ -11,6 +11,7 @@
 require 'wicked_pdf'
 
 Rails.backtrace_cleaner.remove_silencers!
+WickedPdf.silence_deprecations = true
 
 if (assets_dir = Rails.root.join('app/assets')) && File.directory?(assets_dir)
   # Copy CSS file