Linux Mint 18 can’t find header files for ruby
This Tip Shows you How to Solve Linux Mint 18 can’t find header files for ruby at /usr/lib/ruby/include/ruby.h Issue.
And this Issue in Installing Ruby Gems is due to Missing Development Files on Linux Mint System.
sudo apt install ruby-dev
Or if you have Enabled the Ruby PPA:
sudo apt install ruby2.[x]-dev