reformatting error to be py2+py3 compatible

Author: andrewgiessel

folium/utilities.py

@@ -297,8 +297,8 @@ def write_png(array):
 
     array = np.atleast_3d(array)
     if array.shape[2] not in [1, 3, 4]:
-        raise ValueError, "Data must be NxM (mono), " + \
-            "NxMx3 (rgb), or NxMx4 (rgba)"
+        raise ValueError("Data must be NxM (mono), " + \
+            "NxMx3 (rgb), or NxMx4 (rgba)")
 
     # normalize to uint8 if it isn't already
     if array.dtype != 'uint8':