Ruby.h no such file or directory parser.o Error

Using Rails 3.1 and a new application, bundle installI got the following errors at startup

Installing json<1.7.3> with native extensions
Error: Failed to build gem native extension.
Creating Makefile
generating parser-i386-mingw32.def
compiling parser.c
In file included from parser.rl:1:0:../fbuffer/fbuffer.h:6:18: fatal error: ruby.h: No such file or directory
compilation terminated.
make: *** [parser.o] Error 1

I successfully installed DevKit and changed $PATHas shown below:

C:\Users\peter\Downloads\make-3.81-bin\bin;
C:\Users\peter\Working\Ruby-devKit\mingw\bin;
C:\Users\peter\Working\Ruby-devKit;
C:\Program Files\ImageMagick-6.7.8-Q16;
C:\Ruby193\bin;C:\Ruby\bin;