changelog: adds releaste date for 5.1.0-beta1

CHANGELOG.md

-# 5.1.0 (unreleased)
+# 5.1.0-beta1 (2018-04-20)
 
 * **MSSQL**: New Microsoft SQL Server data source [#10093](https://github.com/grafana/grafana/pull/10093), [#11298](https://github.com/grafana/grafana/pull/11298), thx [@linuxchips](https://github.com/linuxchips)
 * **Prometheus**: The heatmap panel now support Prometheus histograms [#10009](https://github.com/grafana/grafana/issues/10009)