Set Thin and ExtraLight styles to 250 and 275 in source Instance Custom Parameters

com.google.fonts/check/020:

```
GlyphsApp users must set a Instance Custom Parameter for the Thin and ExtraLight styles to 250 and 275, so that if OTFs are exported then it will not blur on Windows.
```

I had this, but removed it when I couldn't find the reasoning to include it. Looks like I should re-add it.