Use plain pytest for Python 2.7

Author: tswast

.travis.yml

@@ -49,7 +49,10 @@ install:
     fi
 
 script:
-  - if [[ $PYTHON == '2.7' ]]; then nox -s test ; fi
+  - if [[ $PYTHON == '2.7' ]]; then
+      pip install -r ci/requirements-2.7-0.19.2.pip ;
+      pytest tests/unit ;
+    fi
   - if [[ $PYTHON == '3.5' ]]; then nox -s test_earliest_deps ; fi
   - if [[ $PYTHON == '3.6' ]] && [[ "$PANDAS" == "MASTER" ]]; then nox -s test_latest_deps ; fi
   - REQ="ci/requirements-${PYTHON}-${PANDAS}" ;