improve gh-pages script

- create a shallow clone with a history truncated
- run on postpublish

package.json

@@ -53,8 +53,9 @@
     "coveralls": "istanbul cover _mocha --report lcovonly -- -R dot && cat ./coverage/lcov.info | coveralls",
     "travis": "npm run codestyle-and-test && npm run coveralls",
     "browserify": "browserify -t package-json-versionify --standalone csso lib/index.js | uglifyjs --compress --mangle -o dist/csso-browser.js",
-    "gh-pages": "git clone -b gh-pages https://github.com/css/csso.git .gh-pages && npm run browserify && cp dist/csso-browser.js .gh-pages/ && cd .gh-pages && git commit -am \"update\" && git push && cd .. && rm -rf .gh-pages",
-    "prepublish": "npm run browserify"
+    "gh-pages": "git clone --depth=1 -b gh-pages https://github.com/css/csso.git .gh-pages && npm run browserify && cp dist/csso-browser.js .gh-pages/ && cd .gh-pages && git commit -am \"update\" && git push && cd .. && rm -rf .gh-pages",
+    "prepublish": "npm run browserify",
+    "postpublish": "npm run gh-pages"
   },
   "dependencies": {
     "css-tree": "1.0.0-alpha25"