CI: update azure VM image
Email from Azure support

> On March 23, 2020, we’ll be removing the following Azure Pipelines hosted images:
>
> * Windows Server 2012R2 with Visual Studio 2015, a.k.a vs2015-win2012r2
> * macOS X High Sierra 10.13, a.k.a macOS-10.13
> * Windows Server Core 1803, a.k.a win1803

> If you use YAML pipelines, update your pipeline by editing its YAML file.
> 
>  
> 
>   jobs:
> 
>     - job: Windows
> 
>       pool:
> 
>         vmImage: 'vs2017-win2016'
> 
>       steps:
> 
>       - script: echo hello from Windows