Make codegen to work with python3.

 * Python 3 is now default for some Linux distros.
Update INSTALL.linux.md

 * Python 2 is not required anymore.
 * Fix nits.
Make README.md to link to the Rust-CI generated rustic pages.
Merge branch 'rust-servo' into rust-0.8: Make README.md to link to the Rust-CI generated rustic pages.