Upgrade Babel to 7.0.0-beta.56 (#216)

Summary:
**Summary**

This pull request bumps Babel to 7.0.0-beta.56 to incorporate the latest bugfixes from Babel.

**Test plan**

One way to test is to add a simple TypeScript file with a type as the default export:

```typescript
export const test = 1
export default interface A {}
```

and then import that file from any other file:

```js
import { test } from './typescript-test'
```

Before this would throw an error, but with the latest Babel this is now fixed.
Pull Request resolved: facebook/metro#216

Reviewed By: mjesun

Differential Revision: D9216331

Pulled By: rafeca

fbshipit-source-id: a48e87c3c6b2902be410395b1b4afab63cb88fec

package.json

@@ -201,7 +201,7 @@
     "yargs": "^9.0.0"
   },
   "devDependencies": {
-    "@babel/core": "7.0.0-beta.54",
+    "@babel/core": "7.0.0-beta.56",
     "async": "^2.4.0",
     "babel-eslint": "9.0.0-beta.2",
     "babel-generator": "^6.26.0",