Rails 3, ошибка при установке pg gem

При попытке установить pg gem я получил следующую ошибку:

alex@ubuntu:~/RubymineProjects/Tiger Skills$ gem install pg
ERROR:  While executing gem ... (Gem::FilePermissionError)
You don't have write permissions into the /var/lib/gems/1.9.1 directory.
alex@ubuntu:~/RubymineProjects/Tiger Skills$ sudo gem install pg
Building native extensions.  This could take a while...
ERROR:  Error installing pg:
    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:2:in `<main>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/pg-0.15.1 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/pg-0.15.1/ext/gem_make.out

файл gem_make.out:

/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:2:in `<main>'

Уже пытался решить с помощью

$ sudo apt-get install postgresql-client libpq5 libpq-dev

тогда:

$ sudo gem install pg