Updating to a more modern Leaflet in demo

[efosler]

Thanks for a cool plug-in. Saved me a bunch of time!

To consider:
The current demo uses 1.0.0 as the leaflet base. I ran into an issue where LayerGroups did not set options correctly in 1.0.0 (which is needed for setting timestamps correctly). Updating to 1.3.4 fixed the issue for me (this was corrected in leaflet sometime between 1.0.0 and 1.3.4), so perhaps moving the demo up to the current version might prevent others from running into the same bug.