ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux]
sudo gem install travis
Building native extensions. This could take a while...
ERROR: Error installing travis:
ERROR: Failed to build gem native extension.
/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
It looks like you're using Ruby 1.9.1 installed via the system. In order to fix that error, you should install the
sudo apt-get install ruby1.9.1-dev.
Since the gem uses native extensions, you should also do a
sudo apt-get install build-essentials
However, I would strongly recommend using RVM and using the RVM ruby, since it pulls all most of the dev dependencies for your system and you don't have native extension problems so often.