moved pandas to be a required dependency in tox.ini

Author: emmanuelle

Diff for: packages/python/plotly/tox.ini

@@ -59,14 +59,14 @@ deps=
     pytz==2016.10
     retrying==1.3.3
     pytest==3.5.1
+    pandas==0.19.2
     backports.tempfile==1.0
     optional: --editable=file:///{toxinidir}/../plotly-geo
     optional: numpy==1.11.3
     optional: ipython[all]==5.1.0
     optional: ipywidgets==7.2.0
     optional: ipykernel==4.8.2
     optional: jupyter==1.0.0
-    optional: pandas==0.19.2
     optional: scipy==0.18.1
     optional: shapely==1.6.4
     optional: geopandas==0.3.0