linux azure-pipelines update

azure-pipelines.yml

@@ -25,7 +25,7 @@ variables:
 resources:
   containers:
   - container: LinuxContainer
-    image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-14.04-cross-0cd4667-20170319080304
+    image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-0cd4667-20170319080304
 
 stages:
 - stage: build
@@ -130,7 +130,7 @@ stages:
           
       - job: Linux
         pool:
-          name: Hosted Ubuntu 1604
+          vmImage: ubuntu-18.04
         container: LinuxContainer
         strategy:
           matrix: