Modernize the starting a real project guide

[lexidor]

https://docs.hhvm.com/hack/getting-started/starting-a-real-project

• TravisCI -> GA
• composer require hsl -> use other project
• hint towards using ext_watchman for autoloading (but recommend hhvm-autoload, as it is more beginner friendly)
• Rerun hh_client on bad example to get the new error layout
• Clean up hhast's hhconfig allow list for suppression comments (hsl is built-in, but FIXME for hsl is still curl'ed into new projects)