RUBY-135 Use yajl in JSON compatibility mode for j2bson

bin/j2bson

@@ -29,11 +29,10 @@ require 'bson'
 # Note that, at the moment, this will not properly round-trip
 # in all cases from the output generated by b2json.
 begin
-require 'json/pure'  # broken with 'json/ext'
+require 'yajl/json_gem'  # broken with 'json/ext'
 rescue LoadError
-  puts "This script requires json/pure. Please install one of the following:"
-  puts "  gem install json_pure" 
-  puts "  gem install json" 
+  puts "This script requires yajl. Please install the following:"
+  puts "  gem install yajl"
   Process.exit
 end