Update the version number

Because I forgot. Again. :(

Cargo.toml

 [package]
 
 name = "tiled"
-version = "0.3.0"
+version = "0.4.0"
 description = "A rust crate for loading in maps created by the Tiled editor"
 repository = "https://github.com/mattyhall/rs-tiled.git"
 # documentation = "http://rust-ci.org/mattyhall/rs-tiled/doc/tiled/"