Update html5ever

Follow-up to rust-lang#664.
The only crate from rust-lang#664
that hasn't been updated in `comrak`, which introduced several more
transitive dependencies in later version.

This gets rid of a version of quote and syn.

Author: jyn514

Cargo.lock

@@ -25,7 +25,7 @@ badge = { version = "0", path = "src/web/badge" }
 failure = "0.1.3"
 comrak = { version = "0.3", default-features = false }
 toml = "0.5"
-html5ever = "0.22"
+html5ever = "0.24"
 schemamama = "0.3"
 schemamama_postgres = "0.2"
 rusoto_s3 = "0.40"