Make the api docs build on master automatically

Better written as `if [[ -n "${TOX_ENV}" ]] ; then tox -e $TOX_ENV ; fi` (`-n` will check for non-zero length iirc)

```bash
~ export TOX_ENV=
~ [[ -z "${TOX_ENV}" ]] && echo "FOO"
FOO
~ [[ -n "${TOX_ENV}" ]] && echo "FOO"
~ export TOX_ENV="py27"
~ [[ -n "${TOX_ENV}" ]] && echo "FOO"
FOO
~ [[ -z "${TOX_ENV}" ]] && echo "FOO"
```