Adds depend_on_assets directives

[bdarfler]

Plays nicely with sprockets_better_errors and Rails 4.1
which require depend_on_assets directives if the asset depends
on other assets. In this case the leaflet.js.erb depends on the
marker images.

[bdarfler]

Looks like this is not the official repo?

[tarmo]

@bdarfler Yeah, correct one is Leaflet/Leaflet

[bdarfler]

Really? even for the rails gem? I opened a PR against https://github.com/axyjo/leaflet-rails

[tarmo]

Ah right, I forgot the leaflet-rails repo was originally based on Leaflet/Leaflet. It does look like axyjo/leaflet-rails is currently the most active one, I only created my own fork for a one-off need and don't have plans for continuing to maintain it.