Update atom crate version dependency (#6)

Also bump this crate's version to 0.3.0, because atom's types are
exposed in syndication's public API, which will force clients to update
atom if they depend on it and if they need to interoperate with
syndication.

[breaking-change]

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "syndication"
-version = "0.2.0"
+version = "0.3.0"
 authors = ["Tom Shen <[email protected]>"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/tomshen/rust-syndication"
@@ -9,5 +9,5 @@ keywords = ["atom", "blog", "feed", "rss", "syndication"]
 exclude = ["test-data/*"]
 
 [dependencies]
-atom_syndication = "0.1"
+atom_syndication = "0.2"
 rss = "0.3"