ci: improve appveyor config (#642)

appveyor.yml

@@ -1,29 +1,41 @@
 branches:
   only:
     - master
+    - next
 init:
   - git config --global core.autocrlf input
+cache:
+  - node_modules
+  - '%APPDATA%\npm-cache'
 environment:
   matrix:
+    - nodejs_version: '6'
+      webpack_version: latest
+      job_part: test
     - nodejs_version: '8'
       webpack_version: latest
       job_part: test
-    - nodejs_version: '6'
+    - nodejs_version: '10'
       webpack_version: latest
       job_part: test
-    - nodejs_version: '8'
-      webpack_version: 4.0.0
+    - nodejs_version: '11'
+      webpack_version: latest
       job_part: test
+    - nodejs_version: '6'
+      webpack_version: next
+      job_part: next
 build: 'off'
 matrix:
   fast_finish: true
 install:
   - ps: Install-Product node $env:nodejs_version x64
   - npm i -g npm@latest
-  - npm install
+  - npm ci
+  - npm i -g @webpack-contrib/tag-versions
 before_test:
   - cmd: npm install webpack@%webpack_version%
 test_script:
   - node --version
   - npm --version
-  - cmd: npm run appveyor:%job_part%
+  - cmd: FOR /F %%I in ('compver --name webpack --gte %webpack_version% --lt latest') do SET COMPARED_VERSION_RESULT=%%I
+  - cmd: IF %COMPARED_VERSION_RESULT% NEQ -1 (npm run appveyor:test) ELSE (ECHO "Next is older than Latest - Skipping Canary Suite")