Require aws-sdk in Ruby irb produces error

I tried using: require 'aws-sdk-core in irb on my centos machine, but it shows error:

2.1.5 :001 > require 'aws-sdk-core'

LoadError: cannot load such file -- aws-sdk-core

When I did gem list --local It shows the list as:

aws-s3 (0.6.3)
aws-sdk (2.1.19)
aws-sdk-core (2.1.19)
aws-sdk-resources (2.1.19)
bigdecimal (1.2.4)
builder (3.2.2)
bundler (1.10.6)
io-console (0.4.2)
jmespath (1.0.2)....... 

I am not able to figure out the issue, any suggestions would be appreciated. Thanks :)