continuous-build.yml: use new VSTS YAML syntax

build/tfs/continuous-build.yml

-phases:
-- phase: Windows
-  queue: Hosted VS2017
+jobs:
+- job: Windows
+  pool:
+    vmImage: VS2017-Win2016
   steps:
   - template: win32/continuous-build-win32.yml
 
-- phase: Linux
-  queue: Hosted Linux Preview
+- job: Linux
+  pool:
+    vmImage: Ubuntu 16.04
   steps:
   - template: linux/continuous-build-linux.yml
 
-- phase: macOS
-  queue: Hosted macOS Preview
+- job: macOS
+  pool:
+    vmImage: macOS 10.13
   steps:
-  - template: darwin/continuous-build-darwin.yml
\ No newline at end of file
+  - template: darwin/continuous-build-darwin.yml