undeclared production Babel dependency in v0.25.0

The npm package includes a babel dependency that is undeclared in package.json



specifically, line 3 of /lib/index.js:



```js

var _interopRequireDefault = require("@babel/runtime/helpers/builtin/interopRequireDefault");

```



## Steps to reproduce



Just initialize a new npm repo and add `yup` as a dependency - it can't be imported in node (I'm running node v9.11.1 but I don't think that's related)



```

$ yarn init

$ yarn add yup

...

$ node

> require('yup')

Error: Cannot find module '@babel/runtime/helpers/builtin/interopRequireDefault'

    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:548:15)

    at Function.Module._load (internal/modules/cjs/loader.js:475:25)

    at Module.require (internal/modules/cjs/loader.js:598:17)

    at require (internal/modules/cjs/helpers.js:11:18)

```



This appears to be breaking type checking in Flow because it can't resolve the module, although my dev build does appear to work.

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