Merge pull request #805 from dotnet/update-build-yaml

switch to stable 2.1.400 sdk

.vsts.ix-shared.yml

@@ -16,7 +16,7 @@ steps:
   
 - task: DotNetCoreInstaller@0
   inputs:
-    version: '2.1.400-preview-009171'
+    version: '2.1.400'
 
 - task: DotNetCoreCLI@2
   inputs:

.vsts.rx-shared.yml

@@ -16,7 +16,7 @@ steps:
   
 - task: DotNetCoreInstaller@0
   inputs:
-    version: '2.1.400-preview-009171'
+    version: '2.1.400'
 
 - task: MSBuild@1
   displayName: Build System.Reactive.sln