Update build.gradle to support RNv56

[abartolo]

This PR is to support React Native v56. As discussed in #83 I have tested this change and now I am pointing to my fork in order to successfully build my project. From my tests snackbar has been working as intended throughout the entire app.

[abartolo]

@cooperka I tested these changed on react-native-snackbar v0.4.4 and it worked. However, testing on v0.4.7 I am seeing an issue with the following line in the package.json https://github.com/abartolo/react-native-snackbar/blob/265b8a61e23e0510a9aa0d4396370c1c5533bc8f/package.json#L5

Issue
"main": "lib/index.js",

Error on npm start

Loading dependency graph, done.
error: bundling failed: Error: While trying to resolve module `react-native-snackbar` from file `/Users/abc/efg/hij/jlmn/app/src/screens/AppInitializationScreen/AppInitializationScreen.js`, the package `/Users/abc/efg/hij/jlmn/app/node_modules/react-native-snackbar/package.json` was successfully found. However, this package itself specifies a `main` module field that could not be resolved (`/Users/abc/efg/hij/jlmn/app/node_modules/react-native-snackbar/lib/index.js`. Indeed, none of these files exist:

Fix
"main": "src/index.js",

Adding that fix my build did not fail.

Can you confirm that "lib/index.js" is an issue or there is something I am missing on my side?

[cooperka]

Thank you @abartolo! Looks like a robust fix. I'll merge this and test a few more things before release. lib/index.js is generated by the build script, so it won't exist unless you yarn run build first.