Upgrade xml_writer to latest version to allow compiling on stable

[laumann]

Hi! First of all, thanks for this project, it was just what I needed :-) The current version (0.1.0) doesn't compile on stable, because xml_writer doesn't (didn't). This PR just got merged to allow xml_writer to compile on stable, which in turn should allow this crate to compile on stable.

Once the updated xml_writer (I think it's 0.3) in on crates.io, this crate could be updated to use it.