change gl2d_date_axes mock to date string axis range

I still can't tell why this is passing as a test when it
fails for me in the dashboard (because of timezone conversion)

Author: alexcjohnson

test/image/mocks/gl2d_date_axes.json

@@ -60,8 +60,8 @@
     "layout": {
         "xaxis": {
             "range": [
-                1477434179998,
-                1477434180004
+                "2016-10-25 22:22:59.998",
+                "2016-10-25 22:23:00.004"
             ],
             "autorange": false
         }