fix(babel): Add nativeserializer to packaging step (#227)

When src/NativeSerializer.js was added it wasn't compiled by the npm prepublish script, which only
mentioned src/Bugsnag.js explicitly. This commit ensures anything inside src/ (except the tests)
gets compiled into lib/.

For additional clarity, the argument names for the babel command are now
expanded.

Fixes #226.

Author: fractalwrench

package.json

@@ -13,7 +13,7 @@
     "prop-types": "^15.6.0"
   },
   "scripts": {
-    "prepublish": "mkdir -p lib && babel src/Bugsnag.js -s -o lib/Bugsnag.js",
+    "prepublish": "mkdir -p lib && babel --source-maps --out-dir='lib' --ignore='__tests__' src",
     "test:unit:js": "jest --coverage src/__tests__",
     "test:lint:js": "standard",
     "test:smoke:typescript": "tslint index.d.ts && tsc --strict test/app.ts"