Use appveyor previous VS image

Author: jorgebay

appveyor.yml

@@ -7,7 +7,7 @@ cache:
   - C:\Users\appveyor\.ccm\repository
   - C:\Users\appveyor\deps -> appveyor.ps1
   - C:\Users\appveyor\ccm
-os: Visual Studio 2015
+os: Previous Visual Studio 2015
 platform:
   - Any CPU
 configuration:
@@ -23,7 +23,7 @@ environment:
       CASSANDRA_VERSION: 3.7
       CI_TYPE: INTEGRATION
     - TARGET: netcoreapp1.0
-      CASSANDRA_VERSION: 2.2.5
+      CASSANDRA_VERSION: 3.7
       CI_TYPE: INTEGRATION
     - TARGET: net452
       CASSANDRA_VERSION: 2.2.5