Error about permission when installing

[MoOx]

~/Sync/Development/cssnext/bench  
$ npm i -D ruby-sass
npm WARN package.json cssnext-bench@0.1.0 No repository field.
npm WARN package.json cssnext-bench@0.1.0 No README data

> weak@0.3.3 install /Users/MoOx/Sync/Development/cssnext/bench/node_modules/ruby-sass/node_modules/dnode/node_modules/weak
> node-gyp rebuild

  CXX(target) Release/obj.target/weakref/src/weakref.o
  SOLINK_MODULE(target) Release/weakref.node
  SOLINK_MODULE(target) Release/weakref.node: Finished

> ruby-sass@0.0.3 postinstall /Users/MoOx/Sync/Development/cssnext/bench/node_modules/ruby-sass
> node postinstall.js

Installing required Ruby gems...
$ gem install sass dnode

Fetching: sass-3.4.8.gem (100%)
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.

Error: gem process exited with code 1

npm ERR! Darwin 14.0.0
npm ERR! argv "node" "/usr/local/bin/npm" "i" "-D" "ruby-sass"
npm ERR! node v0.10.33
npm ERR! npm  v2.1.6
npm ERR! code ELIFECYCLE

npm ERR! ruby-sass@0.0.3 postinstall: `node postinstall.js`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the ruby-sass@0.0.3 postinstall script.
npm ERR! This is most likely a problem with the ruby-sass package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node postinstall.js
npm ERR! You can get their info via:
npm ERR!     npm owner ls ruby-sass
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /Users/MoOx/Sync/Development/cssnext/bench/npm-debug.log

~/Sync/Development/cssnext/bench  30s

[astericky]

I get this same error.

[PatrickHeneise]

Same error, please remove the gem install, it has nothing to in an npm routine.

[ridwant]

Same error ! what's the solution?