More tweaks to npm usage on Windows

appveyor.yml

@@ -22,9 +22,9 @@ install:
 - git clone . c:\gopath\src\gopkg.in\urfave\cli.v2
 - go get github.com/urfave/gfmxr/...
 - go get -v -t ./...
-- ps: Install-Product node 6
-- ps: $MarkdownTocExists = Test-Path node_modules\.bin\markdown-toc
-- ps: if ($MarkdownTocExists -eq $False) { npm install markdown-toc }
+- ps: if (Test-Path node_modules\.bin\markdown-toc -eq $False) {
+    npm install markdown-toc
+  }
 
 build_script:
 - python runtests vet