Support for @babel/* packages

I'm trying to use this module with Babel 7.x, but are running into problems which I think are caused by the switch to `@babel/`-scoped packages.

My code:

```js
const modifyBabelPreset = require('modify-babel-preset')

module.exports = modifyBabelPreset('babel-preset-react-native', {
'@babel/plugin-transform-flow-strip-types': false,
'@babel/plugin-transform-typescript': true
})
```

First error:

```text
TypeError: Cannot read property 'withDevTools' of undefined
at module.exports.options (/Users/linus/coding/babel-preset-react-native-typescript/node_modules/babel-preset-react-native/configs/main.js:123:15)
at normalizePreset (/Users/linus/coding/babel-preset-react-native-typescript/node_modules/modify-babel-preset/lib/normalize-preset.js:12:12)
at module.exports (/Users/linus/coding/babel-preset-react-native-typescript/node_modules/modify-babel-preset/index.js:83:21)
```

Applying patch:

```diff
var babelCore;
-try { babelCore = require('babel-core'); } catch(err) {}
+try { babelCore = require('@babel/core'); } catch(err) {}
```

Second error:

```text
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
resolve failed for "@babel": Error: Cannot find module '@babel'
Error: Cannot find module 'babel-plugin-@babel/transform-typescript'
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:594:15)
at Function.Module._load (internal/modules/cjs/loader.js:520:25)
at Module.require (internal/modules/cjs/loader.js:650:17)
at require (internal/modules/cjs/helpers.js:20:18)
at requireBabelPlugin (/Users/linus/coding/babel-preset-react-native-typescript/node_modules/modify-babel-preset/index.js:54:15)
at /Users/linus/coding/babel-preset-react-native-typescript/node_modules/modify-babel-preset/index.js:134:19
at Array.forEach ()
at module.exports (/Users/linus/coding/babel-preset-react-native-typescript/node_modules/modify-babel-preset/index.js:120:29)
```