Fix bad reference to census_shapes.yml using RVM.

...This commit fixes the error *No such file or directory - lib/yml/census_shapes.yml*.

To repo the error:
* Start rails console
* Create an instance of CensusApis::Client
client = CensusApi::Client.new(CENSUS_API_KEY)
* Run command client.sf1('P0010001', 'STATE:06')

You will get "Errno::ENOENT: No such file or directory - lib/yml/census_shapes.yml"

The issue was the path was was trying to look into the project lib directory instead of the gem.