Add complex/spark to PR checklist (#1452)

Author: henryre

.github/pull_request_template.md

@@ -13,4 +13,5 @@ Need help on these? Just ask!
 * [ ] I have read the **CONTRIBUTING** document.
 * [ ] I have updated the documentation accordingly.
 * [ ] I have added tests to cover my changes.
+* [ ] I have run `tox -e complex` and/or `tox -e spark` if appropriate.
 * [ ] All new and existing tests passed.