Gem::Ext::BuildError: ERROR: Failed to build gem native extension for rails version 4.2.6

If you have a Mac or Linux machine try the following command:

sudo apt-get install ruby-dev

If that fails, what is your output for:

gem install json -v '1.8.6'

(please copy and paste all the output).

Similar issue

I'm not sure what will fix it for Mac, but people had positive experiences with brew install ruby.