clean dist before building

Author: Tyler Brandt

packages/optimizely-sdk/package.json

@@ -7,7 +7,7 @@
   "scripts": {
     "test": "mocha ./lib/*.tests.js ./lib/**/*.tests.js ./lib/**/**/*tests.js --recursive",
     "test-xbrowser": "karma start karma.bs.conf.js --single-run",
-    "build-browser-umd": "webpack",
+    "build-browser-umd": "rm -rf dist && webpack",
     "lint": "eslint lib/**",
     "cover": "istanbul cover _mocha ./lib/*.tests.js ./lib/**/*.tests.js ./lib/**/**/*tests.js",
     "coveralls": "npm run cover -- --report lcovonly && cat ./coverage/lcov.info | coveralls",