Use Bundler's .which method instead of the shell for finding wkhtmltopdf

Using the shell to get this information leads to the risk of capturing
warning information from bundler. Using the which method (which has been
around for years) will do the same thing, but will only return the path
and nothing else.

lib/wicked_pdf.rb

@@ -336,7 +336,7 @@ def find_wkhtmltopdf_binary_path
     possible_locations += %w[~/bin] if ENV.key?('HOME')
     exe_path ||= WickedPdf.config[:exe_path] unless WickedPdf.config.empty?
     exe_path ||= begin
-      detected_path = (defined?(Bundler) ? `bundle exec which wkhtmltopdf` : `which wkhtmltopdf`).chomp
+      detected_path = (defined?(Bundler) ? Bundler.which('wkhtmltopdf') : `which wkhtmltopdf`).chomp
       detected_path.present? && detected_path
     rescue StandardError
       nil