fix: freeze sass gem version

It raises warning such as `Warning. Error encountered while saving cache
b05ef7e3c2046a673504ca75a734e6b26e9498a6/_top-bar.scssc: can't dump
anonymous class #<class:0x007fc531ed7a58>` which breaks assets
compilation.

It's a known bug but for now nobody knows when it will be fixed.