fix(react): replace invalid characters in chunk name

We're using base64 to generate short chunk names, unfortunately this
means that we need to replace some characters which are invalid
characters in file names.
We replace `/` with `-`, `+` with `_` and `=` with an empty string.
Replacing `=` with an empty string shouldn't occur in real life,
because we're only using the first four characters.

packages/react/import-component/babel.js

@@ -81,7 +81,12 @@ module.exports = ({ types: t }) => ({
           const hasher = crypto.createHash('md4');
           hasher.update(nodePath.relative(this.opts.rootDir, state.filename));
           hasher.update(importedComponent);
-          const hash = hasher.digest('base64').slice(0, 4);
+          const hash = hasher
+            .digest('base64')
+            .replace('/', '-')
+            .replace('+', '_')
+            .replace('=', '')
+            .slice(0, 4);
 
           t.addComment(
             argPath.node,