Adds new secure env keys for the canonical repository

:information_desk_person: I'd neglected to note that [Travis CI's secure environment variables are tied to specific repositories](http://about.travis-ci.org/docs/user/build-configuration/#Secure-environment-variables). This change adds in secure variables for the `bigcommerce/bigcommerce-api-php` repository.

As the smoke tests are the only tests that require these variables, they may need to be isolated to only run on the canonical repository. This will prevent false failures for pull requests coming in from other forks.