Re-fix require statement to use correct name

This was fixed via https://github.com/avadev/AvaTax-Calc-SOAP-Ruby/commit/f0e71a4
but I think it got undone while resolving merge conflicts in https://github.com/avadev/AvaTax-Calc-SOAP-Ruby/commit/81eb53b
Merge pull request #3 from jordan-brough/re-fix-requires

Re-fix require statement to use correct name