Override any debug flags that may be present

[mvz]

This is a minimal fix for #200. It may be too simple and there may be reasons not to fix things this way (one of them being that it makes debugging while developing sassc-ruby harder).

[marcelolx]

@glebm Can we get some eyes on this?

[marcelolx]

@bolandrm If we can get your eyes on this as well, no rush since we can use the forked repo ✌️