to_str() -> to_string()

to_str() was changed to to_string() everywhere after https://github.com/rust-lang/rust/pull/15493

In iban.rs to_str() still works (and to_string() breaks the program) but I haven't been able to understand why
I'll push the commit with the updated iban.rs and will squash them together tomorrow, just to see if tomorrow Travis can build it too?
There seems to be still a bit of activity on Rust on the to_str / to_string front and I would like for it to solidify. Yesterday iban.rs compiled with to_str and into_str, today it needs to_string and into_string...
What do you think?