Bootstrap path in cache command

In Laravel 5.2.23 (I think), a new `bootstrapPath()` method was added to the application container to allow for updating where the bootstrap directory lives. Today I ran into errors when using the `api:routes` command because my bootstrap directory isn't at the root like the cache command expects.

I was about to send a pull request to update the cache command to use the new method, but when I saw the Composer file referenced some 5.1 components, I wasn't sure it would be accepted since the method was only recently added.

Is there a chance of being able to use the `bootstrapPath()` method instead of hard-coding the path in? If so, would you want me to check for a Laravel version or for the existence of the `bootstrapPath()` method?

Appreciate your feedback!