Silence Prince message fin|success

Changes how the Prince command is executed and stdout is capture in
order to hide the message `fin|success`.

lib/princely/pdf.rb

@@ -1,3 +1,4 @@
+require 'open3'
 require 'timeout'
 
 module Princely
@@ -69,9 +70,8 @@ def executable_options
     def pdf_from_string(string, output_file = '-')
       with_timeout do
         pdf = initialize_pdf_from_string(string, output_file, {:output_to_log_file => false})
-        pdf.close_write
         result = pdf.gets(nil)
-        pdf.close_read
+        pdf.close
         result.force_encoding('BINARY') if RUBY_VERSION >= "1.9"
 
         result
@@ -107,9 +107,10 @@ def initialize_pdf_from_string(string, output_file, options = {})
       log_command path if options[:log_command]
 
       # Actually call the prince command, and pass the entire data stream back.
-      pdf = IO.popen(path, "w+")
-      pdf.puts string
-      pdf
+      stdin, stdout = Open3.popen3(path)
+      stdin.puts string
+      stdin.close
+      stdout
     end
 
     def log_command(path)