Can't build TTF and variable

I'm sure this used to work...

```
$ fontmake -o ttf variable -g sources/NotoSerifOriya.glyphs
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.classes:Parsing "sources/NotoSerifOriya.glyphs" file into
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern1.gadaorya found in kerning rules.
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern1.nnannaUIorya found in kerning rules.
fontmake: Error: In 'sources/NotoSerifOriya.glyphs' -> 'master_ufo/NotoSerifOriya.designspace': Generating fonts from Designspace failed: save_otfs() got an unexpected keyword argument 'optimize_gvar'
```