Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby -r ./siteconf20150404-6-1pa98vy.rb extconf.rb
/usr/lib/ruby/2.2/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/lib/ruby/2.2/rubygems/core_ext/kernel_require.rb:54:in `require'
	from extconf.rb:1:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /usr/lib/ruby/gems/2.2/gems/unf_ext-0.0.6 for inspection.
Results logged to /usr/lib/ruby/gems/2.2/extensions/x86_64-linux/2.2/unf_ext-0.0.6/gem_make.out
An error occurred while installing unf_ext (0.0.6), and Bundler cannot continue.
Make sure that `gem install unf_ext -v '0.0.6'` succeeds before bundling.
INFO[0028] The command [/bin/sh -c bundle install --without development test] returned a non-zero code: 5