Fixes: #74

This PR allows running the JS Rollbar client without the NativeClient native binding. For unejected Expo apps, or any other JS only React Native app, just npm/yarn install and add to your project:

import { Client, Configuration } from 'rollbar-react-native'
const config = new Configuration('POST_CLIENT_ITEM_ACCESS_TOKEN', {
  logLevel: 'info'
});
const rollbar = new Client(config);

Notes

This no longer throws when the native binding is missing, and omits the message about react-native link. Since RN 0.60, react-native link is no longer used (native dependencies are autolinked), so this should be fine. Especially since JS-only apps are becoming more common.

I first looked at whether Expo apps could just import the react-native target of Rollbar.js (which is what rollbar-react-native wraps for javascript support.) This would introduce dependencies (buffer, promise) that are currently handled by the rollbar-react-native wrapper. By continuing to use rollbar-react-native, the config is simpler and no new dependencies are added to rollbar.js.

For React Native Web apps, or any app that isn't a native app and runs in a browser, the browser target of Rollbar.js should be used instead. Those environments will use the browser's onerror and won't have React Native's ErrorUtils.