-middleman server ==> cannot load such file -- middleman-core (LoadError)

middleman server throwing me an error that is unknown to me can you please help me on below error how can i resolve this issue
i’m using window 8.1

Directories in my PC.
Ruby:
C:\Ruby22-x64
and
DevKit:
C:\Ruby22-x64\RubDevKit

and i am creating Project in C:\ middleman init PROJECTNAME

ruby -v

ruby 2.2.4p230 (2015-12-16 revision 53155) [x64-mingw32]

and Dev kit

DevKit-mingw64-64-4.7.2-20130224-1432-sfx