adjusted appveyor config to test different environments

Author: mschilli87

appveyor.yml

@@ -12,6 +12,24 @@ install:
 
 # Adapt as necessary starting from here
 
+environment:
+  global:
+    WARNINGS_ARE_ERRORS: 1
+
+  matrix:
+    - R_VERSION: oldrel
+      R_ARCH: i386
+    - R_VERSION: oldrel
+      R_ARCH: x64
+    - R_VERSION: release
+      R_ARCH: i386
+    - R_VERSION: release
+      R_ARCH: x64
+    - R_VERSION: devel
+      R_ARCH: i386
+    - R_VERSION: devel
+      R_ARCH: x64
+
 build_script:
   - travis-tool.sh install_deps