Remove postpublish CI trigger

Testing prebuilts after release on electron was helpful in the early days of using `prebuild`.

For now this doesn't add any value, so let's remove it.

Author: lgeiger

package.json

@@ -29,7 +29,6 @@
     "install": "node scripts/prebuild-install.js || (node scripts/preinstall.js && node-gyp rebuild)",
     "prebuild": "prebuild --all --strip",
     "build:docs": "jsdoc -R README.md -d docs lib/*.js",
-    "postpublish": "./scripts/trigger_travis_build.sh",
     "test": "mocha --expose-gc --slow 300",
     "test:electron": "electron-mocha --slow 300",
     "precoverage": "nyc npm run test",