Add default Dev15 location

SetDevCommandPrompt.cmd

@@ -3,6 +3,9 @@
 :: prefer building with Dev15
 
 set CommonToolsDir=%VS150COMNTOOLS%
+
+:: VS150COMNTOOLS is not set globally any more, so fall back to default preview location before trying Dev14
+if not exist "%CommonToolsDir%" set CommonToolsDir=%ProgramFiles(x86)%\\Microsoft Visual Studio\\VS15Preview\\Common7\\Tools\\
 if not exist "%CommonToolsDir%" set CommonToolsDir=%VS140COMNTOOLS%
 if not exist "%CommonToolsDir%" exit /b 1
 

netci.groovy

@@ -202,7 +202,6 @@ commitPullList.each { isPr ->
       batchFile("""set TEMP=%WORKSPACE%\\Binaries\\Temp
 mkdir %TEMP%
 set TMP=%TEMP%
-set VS150COMNTOOLS=%ProgramFiles(x86)%\\Microsoft Visual Studio\\VS15Preview\\Common7\\Tools\\
 .\\cibuild.cmd /debug /testVsi""")
     }
   }