Change prepublish script for prepublishOnly

package.json

@@ -54,7 +54,7 @@
  "travis": "nyc npm run lint-and-test && npm run coveralls",
  "hydrogen": "node --trace-hydrogen --trace-phase=Z --trace-deopt --code-comments --hydrogen-track-positions --redirect-code-traces --redirect-code-traces-to=code.asm --trace_hydrogen_file=code.cfg --print-opt-code bin/csso --stat -o /dev/null",
  "gh-pages": "git clone --depth=1 -b gh-pages https://github.com/css/csso.git .gh-pages && npm run build && cp dist/csso.min.js .gh-pages/ && cd .gh-pages && git commit -am \"update\" && git push && cd .. && rm -rf .gh-pages",
- "prepublish": "npm run build",
+ "prepublishOnly": "npm run build",
  "postpublish": "npm run gh-pages"
  },
  "dependencies": {