@import '~bulma/sass/utilities/variables'; is no longer valid

due to recent update to bulma/sass/utilities, it should be now replaced with

```
@import '~bulma/sass/utilities/initial-variables';
@import '~bulma/sass/utilities/derived-variables';
```

BTW, it's fixed by https://github.com/vue-bulma/tooltip/pull/12/files (that's exactly what it was intended to fix anyway)