Run on Rust stable
When the "unstable" Cargo feature is given, html5ever’s build script will use html5ever_macros as a library to generate or update `src/tree_builder/rules.expanded.rs`, which is part of the source tree.
(This feature also enables the corresponding features of tendril and string-cache.)
html5ever_macros is not a plugin anymore, but it still uses libsyntax and still behaves a lot like a syntax extension.
When "unstable" is *not* given, the build script checks that rules.expanded.rs is up to date (based on a hash of rules.rs) and fails if it’s not.
Tests also run on stable Rust on Travis-CI, but unfortunately most of them are disabled as they dynamically generated using internals of the `test` crate.
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