Redmine

I've searched everywhere and tried everything and had no luck with this yet...

First of all, I'm trying to follow this "tutorial" here: http://www.redmine.org/projects/redmine/wiki/HowTo_Install_Redmine_on_Debian_Squeeze_with_Postgresql_Ruby-on-Rails_and_Apache2-Passenger

I'm on Debian, trying to install rails through the command "gem install -v=2.3.5 rails"

but I keep getting this error:

ERROR: While executing gem... (NameError)
uninitialized constant Gem::Installer::ENV_PATHS

however I've tried installing, removing, reinstalling and so forth packages ruby, rubygems and rake...

the versions of these softwares are: - ruby(4.2) - rubygems(1.3.5-1ubuntu2) - rake (0.8.7-1)

I've also tried installing rails simply through "gem install rails" but when I do that I get this other error:

ERROR: Error installing rails:
ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb
extconf.rb:13:in `require' no such file to load -- mkmf (LoadError)
from extconf.rb:13

Gem files will remain installed in /usr/local/rvm/gems/ruby-1.9.3-p286/gems/atomic-1.1.14 for inspection.
Results logged to /usr/local/rvm/gems/ruby-1.9.3-p286/gems/atomic-1.1.14/ext/gem_make.out

Please help this is really horrible and I've tried everything... Thanks a lot in advance ;)