Make vega-encode depends on Vega-scale 2.1 or upper
Make vega-encode depends on Vega-scale 2.1 or upper

Otherwise compiling Vega and Vega-encode in depending projects may get the following warnings

```

WARNING in ./~/vega-encode/src/ticks.js
24:36-48 "export 'timeInterval' was not found in 'vega-scale'

WARNING in ./~/vega-encode/src/ticks.js
25:31-42 "export 'utcInterval' was not found in 'vega-scale'
```

vega/vega,vega/vega,lgrammel/vega,vega/vega,vega/vega