chore: add prepare script (googleapis#163)

Ensure that javascript output exist before the package gets published.

Author: ofrobots

package.json

@@ -69,7 +69,8 @@
  "test": "npm run lint && npm run build && npm run check-format && npm run coverage",
  "coveralls": "istanbul cover -x 'apis/**' _mocha --report lcovonly -- --reporter spec --timeout 4000 && cat coverage/lcov.info | coveralls",
  "build": "tsc",
- "clean": "rm -rf lib test types"
+ "clean": "rm -rf lib test types",
+ "prepare": "npm run build"
  },
  "license": "Apache-2.0"
 }