Update to remove ~

I hit the following errors when running 'make all':

```
mkdir -p build/
rustc src/codegen/main.rs --out-dir=build
src/codegen/main.rs:40:57: 40:58 error: obsolete syntax: `~` notation for owned pointers
src/codegen/main.rs:40 pub fn get_writer(output_dir: &Path, filename: &str) -> ~Writer {
                                                                               ^
note: use `Box<T>` in `std::owned` instead
src/codegen/main.rs:44:24: 44:30 error: obsolete syntax: `~` notation for owned pointer allocation
src/codegen/main.rs:44         Ok(writer) => ~writer as ~Writer,
                                              ^~~~~~
note: use the `box` operator instead of `~`
src/codegen/main.rs:44:34: 44:35 error: obsolete syntax: `~` notation for owned pointers
src/codegen/main.rs:44         Ok(writer) => ~writer as ~Writer,
                                                        ^
error: aborting due to 3 previous errors
```

I fixed it by updating `src/codegen/main.rs` as the compiler suggested.