Merge pull request pdfkit#88 from daphonz/master

PDFKit cannot find the wkhtmltopdf binary file in the gem wkhtmltopdf-binary when using Bundler

lib/pdfkit/configuration.rb

@@ -17,7 +17,7 @@ def initialize
     end
 
     def wkhtmltopdf
-      @wkhtmltopdf ||= `which wkhtmltopdf`.chomp
+      @wkhtmltopdf ||= (defined?(Bundler) ? `bundle exec which wkhtmltopdf` : `which wkhtmltopdf`).chomp
     end
   end