laravel 5.6.12 not supported in composer.json

[fittachi]

Your requirements could not be resolved to an installable set of packages.

Problem 1
- Can only install one of: laravel/framework[v5.6.12, 5.5.x-dev].
- Can only install one of: laravel/framework[5.5.x-dev, v5.6.12].
- Can only install one of: laravel/framework[5.5.x-dev, v5.6.12].
- backtory/laravel-backtory-storage v1.0.0 requires laravel/framework 5.5.* -> satisfiable by laravel/framework[5.5.x-dev].
- Installation request for backtory/laravel-backtory-storage ^1.0 -> satisfiable by backtory/laravel-backtory-storage[v1.0.0].
- Installation request for laravel/framework (locked at v5.6.12, required as 5.6.*) -> satisfiable by laravel/framework[v5.6.12].