Installing gems results in error

Installing gems results in error

ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux]
gem 2.0.3
sudo gem install travis
Building native extensions. This could take a while...
ERROR: Error installing travis:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot
load such file -- mkmf (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from extconf.rb:4:in `<main>'
Gem files will remain installed in /var/lib/gems/1.9.1/gems/ffi-1.9.0 for
inspection.
Results logged to /var/lib/gems/1.9.1/gems/ffi-1.9.0/ext/ffi_c/gem_make.out
Also encountering this when trying to install other gems. I'm not a Ruby
user, and googling the error did not get me any working solution.