Use Blender2.80 stable build for travis CI

Blender API is keep changing in Blender master branch, which makes it
hard to do regression tests. Here, switch back to Blender2.80 stable
for all the testing. Will consider testing with up-to-date API when
Blender2.81 releases.

io_scene_godot/converters/material/material.py

@@ -89,12 +89,9 @@ def generate_material_resource(escn_file, export_settings, bl_object,
             logging.error(
                 "%s, in material '%s'", str(exception), material.name
             )
-
-    elif export_settings['material_mode'] == 'SPATIAL':
+    else:  # Spatial Material
         mat = export_as_spatial_material(material_rsc_name, material)
 
-    assert mat is not None
-
     # make material-object tuple as an identifier, as uniforms is part of
     # material and they are binded with object
     return escn_file.add_internal_resource(mat, (bl_object, material))