Error When installing Nokogiri gem - 'no such file to load -- mkmf (LoadError)'

So basically when I try to install nokogiri gem using 'gem install nokogiri', I get the following error:

root@ubuntu:/home# gem install nokogiri
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
 ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb
extconf.rb:5:in 
 `require': no such file to load -- mkmf (LoadError)
    from extconf.rb:5


Gem files will remain installed in 
/usr/lib/ruby/gems/1.8/
  gems/nokogiri for inspection.
Results logged to 
/usr/lib/ruby/gems/1.8/
  gems/nokogiri/ext/nokogiri/gem_make.out

I was able to successfully install nokogiri on Windows and Mac so I was confused as to why it is failing in Ubuntu. I found out that you need to install the ruby1.8-dev to make it work.

sudo apt-get install ruby1.8-dev

If that still does not work try to install the following as well:

sudo apt-get install libxml2 libxml2-dev libxslt1-dev
sudo gem install nokogiri

Then just run the gem install nokogiri:

root@ubuntu:/home# gem install nokogiri -v 1.4.3.1
Building native extensions.  This could take a while...
Successfully installed nokogiri-1.4.3.1
1 gem installed
Installing ri documentation for nokogiri-1.4.3.1...
r
No definition for parse_memory

No definition for parse_file

No definition for parse_with

No definition for get_options

No definition for set_options
Installing RDoc documentation for nokogiri-1.4.3.1...

No definition for parse_memory

No definition for parse_file

No definition for parse_with

No definition for get_options

This above solution will also work if you get the following error message:

Could not open library 'xml2' : xml2: 
cannot open shared object file: No such file or directory. 
Could not open library 'libxml2.so' :
 libxml2.so: cannot open shared object file: No such file or directory (LoadError)

as mentioned on this blog