Ensure lib is built before npm publish

Resolves #33.

Author: jakezatecky

package.json

@@ -20,6 +20,7 @@
     "build": "gulp build",
     "examples": "gulp examples",
     "gh-deploy": "git subtree push --prefix examples/dist origin gh-pages",
+    "prepublish": "in-publish && gulp build",
     "test": "gulp test"
   },
   "peerDependencies": {
@@ -48,6 +49,7 @@
     "gulp-mocha": "^4.0.1",
     "gulp-sass": "^3.0.0",
     "gulp-scss-lint": "^0.4.0",
+    "in-publish": "^2.0.0",
     "jsdom": "^9.11.0",
     "mocha": "^3.0.2",
     "react": "^15.4.2",