Error:

user@host:~$ sudo gem install aws-sdk
Building native extensions. This could take a while...
ERROR: Error installing aws-sdk:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.8 extconf.rb
checking for libxml/parser.h... no
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libxml2 is missing. please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing dependencies.

Solve by installing the required libraries

apt-get install libxml2-dev libxslt1-dev

Source