MAINT: add vl2.6 to changelog
### Maintenance
-- update to Vega-Lite 2.5.2; see vega-lite change-logs [2.5.0](https://github.com/vega/vega-lite/releases/tag/v2.5.0) [2.5.1](https://github.com/vega/vega-lite/releases/tag/v2.5.1) [2.5.2](https://github.com/vega/vega-lite/releases/tag/v2.5.2)
+- update to Vega-Lite 2.6.0; see vega-lite change-logs [2.5.0](https://github.com/vega/vega-lite/releases/tag/v2.5.0) [2.5.1](https://github.com/vega/vega-lite/releases/tag/v2.5.1) [2.5.2](https://github.com/vega/vega-lite/releases/tag/v2.5.2) [2.6.0](https://github.com/vega/vega-lite/releases/tag/v2.6.0)
### Backward-incompatible changes
