Update roxmltree requirement from 0.14 to 0.15

Updates the requirements on [roxmltree](https://github.com/RazrFalcon/roxmltree) to permit the latest version.
- [Release notes](https://github.com/RazrFalcon/roxmltree/releases)
- [Changelog](https://github.com/RazrFalcon/roxmltree/blob/master/CHANGELOG.md)
- [Commits](RazrFalcon/roxmltree@v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: roxmltree
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -39,6 +39,6 @@ rustc-hash = { version = "1.1", optional = true }
 
 # Used in COLLADA parsing.
 indexmap = { version = "1.5.2", optional = true, features = ["std"] }
-roxmltree = { version = "0.14", optional = true }
+roxmltree = { version = "0.15", optional = true }
 
 [dev-dependencies]