Hex package health and janitorial work

[josevalim]

@aseigo

Calls to erlang:get_stacktrace() fail at build in a mix project, and there is a message to the use the “new try/get api instead”. So in recent version of jsone there is this:

Could this be the warning as errors option? Note that warnings_as_errors is always disabled for dependencies, which means that a project failing to build locally because of warnings_as_errors will still work as a dependency.