Update wicked_pdf initializer to the latest version

We haven't been updating this file for a long time so, and this version
uses the `configure` method instead of the `config` hash.

We're also adding a few comments already included in previous versions
of wicked pdf that we hadn't copied to our initializer.

config/initializers/wicked_pdf.rb

@@ -8,15 +8,23 @@
 #
 # https://github.com/mileszs/wicked_pdf/blob/master/README.md
 
-WickedPdf.config = {
+WickedPdf.configure do |config|
   # Path to the wkhtmltopdf executable: This usually isn't needed if using
   # one of the wkhtmltopdf-binary family of gems.
-  # exe_path: '/usr/local/bin/wkhtmltopdf',
+  # config.exe_path = '/usr/local/bin/wkhtmltopdf',
   #   or
-  # exe_path: Gem.bin_path('wkhtmltopdf-binary', 'wkhtmltopdf')
+  # config.exe_path = Gem.bin_path('wkhtmltopdf-binary', 'wkhtmltopdf')
+
+  # Needed for wkhtmltopdf 0.12.6+ to use many wicked_pdf asset helpers
+  config.enable_local_file_access = true
 
   # Layout file to be used for all PDFs
   # (but can be overridden in `render :pdf` calls)
-  # layout: 'pdf.html',
-  enable_local_file_access: true
-}
+  # config.layout = 'pdf.html',
+
+  # Using wkhtmltopdf without an X server can be achieved by enabling the
+  # 'use_xvfb' flag. This will wrap all wkhtmltopdf commands around the
+  # 'xvfb-run' command, in order to simulate an X server.
+  #
+  # config.use_xvfb = true,
+end