rust: Bump semver to 0.15

Prep for some breaking changes.

Cargo.toml

@@ -8,7 +8,7 @@ license = "MIT"
 name = "ostree"
 readme = "rust-bindings/README.md"
 repository = "https://github.com/ostreedev/ostree"
-version = "0.14.0"
+version = "0.15.0"
 
 exclude = [
     "/*.am", "/apidoc", "/autogen.sh", "/bash", "/bsdiff",