add missing standalone flag

Author: stoeffel

package.json

@@ -40,7 +40,7 @@
     "coverage": "istanbul cover ./node_modules/mocha/bin/_mocha  -- --report=lcov --ui=qunit tests",
     "build": "npm run build:clean && npm run build:bundle && npm run build:min",
     "build:clean": "rm -rf dist",
-    "build:bundle": "mkdir dist && browserify index.js -o dist/underscore.string.js -p browserify-header",
+    "build:bundle": "mkdir dist && browserify index.js -o dist/underscore.string.js -p browserify-header -s s",
     "build:min": "uglifyjs dist/underscore.string.js -o dist/underscore.string.min.js --comments",
     "release": "npm test && npm run release:version && npm run build && npm run release:push",
     "release:version": "node scripts/bump-version.js",