While attempting to install a ruby gem you may get this error:
Building native extensions. This could take a while... ERROR: Error installing rails: ERROR: Failed to build gem native extension. /usr/bin/ruby1.8 extconf.rb extconf.rb:36:in `require': no such file to load -- mkmf (LoadError) from extconf.rb:36
To resvole this:
apt-get install ruby1.9.1-dev