fix #998 : prevent caching of route callbacks

[vorban]

This addresses #998 .

Before: php artisan route:cache serializes through reflection the route callbacks, hardwiring the execution context's absolute path to flux's asset files (ex: /Users/vorban/repositories/...) because of __DIR__ resolution.

After: php artisan route:cache serializes references to the route actions, preventing any resolution of __DIR__ before runtime.

I've tested it locally. The routes are listed by artisan route:list, and artisan route:cache correctly serialized references to the functions.

After a route:cache, going to any page that uses the blade directives like @fluxStyles works and the files are correctly fetched.

I'd prefer this to be thoroughly reviewed, as I failed to find the contribution guidelines and test suites to run.