Map Dylan Smith to Dylan Thacker-Smith due to change of last name.

Currently my commits are split between http://contributors.rubyonrails.org/contributors/dylan-smith/commits and http://contributors.rubyonrails.org/contributors/dylan-thacker-smith/commits since I changed my last name from Smith to Thacker-Smith.

E.g. the commit from my first rails pull request (https://github.com/rails/rails/pull/6759/commits) contains the first commit in http://contributors.rubyonrails.org/contributors/dylan-smith/commits, and my last pull request (https://github.com/rails/rails/pull/16069/commits) contains the last commit in http://contributors.rubyonrails.org/contributors/dylan-thacker-smith/commits, yet both pull requests were opened using my github user account.