More tweaks to how ./runtests toc runs on Windows

appveyor.yml

@@ -21,9 +21,7 @@ install:
 - git clone . c:\gopath\src\gopkg.in\urfave\cli.v2
 - go get github.com/urfave/gfmxr/...
 - go get -v -t ./...
-- npm bin
-- ps: $MarkdownTocExists = Test-Path node_modules\.bin\markdown-toc
-- ps: if ($MarkdownTocExists -eq $False) { cmd /c npm install markdown-toc }
+- if not exist node_modules\.bin\markdown-toc npm install markdown-toc
 
 build_script:
 - python runtests vet

runtests

@@ -94,8 +94,10 @@ def _migrations():
 
 @_target
 def _toc():
-    npm_bindir = check_output(['npm', 'bin']).strip()
-    _run(['node', os.path.join(npm_bindir, 'markdown-toc'), '-i', 'README.md'])
+    _run([
+        'node', os.path.join('node_modules', '.bin', 'markdown-toc'),
+        '-i', 'README.md'
+    ])
     _run(['git', 'diff', '--quiet'])