add prepublish instruction to build on npm pubish/install

Author: Antonio Tenorio Fornés

package.json

@@ -15,6 +15,7 @@
     "test:browser": "gulp test:browser",
     "lint": "aegir-lint",
     "build": "gulp build",
+    "prepublish": "gulp build",
     "release": "gulp release",
     "release-minor": "gulp release --type minor",
     "release-major": "gulp release --type major",