Change the folder from "latest" to "dev"

This changes the folder variable to "dev" instead of "latest" when `travis_tag` is empty,
Documenter uses "dev" not instead of "latest", whilst latest is currently symlinked by
Documenter to provide some backward compatibility, it still breaks some things like using `@__NBVIEWER_ROOT_URL__`

src/Literate.jl

@@ -171,7 +171,7 @@ function replace_default(content, sym;
     ## use same logic as Documenter to figure out the deploy folder
     travis_tag = get(ENV, "TRAVIS_TAG", "TRAVIS_TAG")
     if isempty(travis_tag)
-        folder = "latest"
+        folder = "dev"
     else
         # use the versioned directory for links, even for the stable and release-
         # folders since this will never change