Update rustc-serailize dep

Author: oschwald

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "maxminddb"
-version = "0.5.3"
+version = "0.5.4"
 description = "Library for reading MaxMind DB format used by GeoIP2"
 documentation = "http://oschwald.github.io/maxminddb-rust/maxminddb/struct.Reader.html"
 readme = "README.md"
@@ -20,4 +20,4 @@ path = "src/maxminddb/lib.rs"
 [dependencies]
 
 log = "0.3.1"
-rustc-serialize = "0.3.14"
+rustc-serialize = "0.3.15"