I spun up a new instance on Amazon AWS to learn some ruby for one of my classes online and notice I kept getting the below error:

[ec2-user@ip-xx-xx-xx-xx ~]$ sudo gem install 

Fetching: xxxxxxxxxxxxxxxxxxxx (100%)
Successfully xxxxxxxxxxx
Fetching: multi_json-1.10.1.gem (100%)
Successfully installed multi_json-1.10.1
Fetching: rubyzip-1.1.6.gem (100%)
Successfully installed rubyzip-1.1.6
Fetching: ffi-1.9.3.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing xxxxx:
        ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.0 extconf.rb
mkmf.rb can't find header files for ruby at /usr/share/ruby/include/ruby.h

Gem files will remain installed in /usr/local/share/ruby/gems/2.0/gems/ffi-1.9.3 for inspection.
Results logged to /usr/local/share/ruby/gems/2.0/gems/ffi-1.9.3/ext/ffi_c/gem_make.out
[ec2-user@ip-xx-xx-xx-xx ~]$

The above is easy to fix when using ruby gems to install new gems by installing ruby-devel make gcc.

sudo yum install ruby-devel make gcc