add emf to images test suite

Author: michaelbabyn

plotly/tests/test_plot_ly/test_image/test_image.py

@@ -59,7 +59,7 @@ def _test(self):
     return _test
 
 kwargs = {
-    'format': ['png', 'jpeg', 'pdf', 'svg'],
+    'format': ['png', 'jpeg', 'pdf', 'svg', 'emf'],
     'width': [None, 300],
     'height': [None, 300],
     'scale': [None, 5]