Add 'semver' crate to mismatched version whitelist.

clippy uses semver==0.2.x

rustc-version uses semver==0.1.x

djc/rustc-version-rs#5

servo-tidy.toml

@@ -8,7 +8,7 @@ check-ordered-json-keys = [
 
 [ignore]
 # Ignored packages with duplicated versions
-packages = ["bitflags", "lazy_static"]
+packages = ["bitflags", "lazy_static", "semver"]
 # Files that are ignored for all tidy and lint checks.
 files = [
   # Generated and upstream code combined with our own. Could use cleanup