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http://stackoverflow.com/questions/32891965/error-while-executing-gem-errnoeperm-operation-not-permitted

answer from SASS issues.. https://github.com/sass/sass/issues/1768

Apparently with OSX el Capitan, and macOS Sierra there is a new security function that prevents you from modifying system files called Rootless. So you'll have 2 options:

If you install gems on /usr/local/bin there will be no problem because rootless doesn't affect this path.

sudo gem install -n /usr/local/bin GEM_NAME_HERE