Remove comment about now-released @babel/parser@7.5.0.

After updating @babel/parser to version 7.5.0, I can verify that the
monkey-patching changes in PR #230 are no longer necessary, though it
doesn't hurt to leave them in place, since the interception logic is
restricted to versions in the 7.4.x range.

The allowUndeclaredExports option is now required, as anticipated:
babel/babel#9864

lib/parsers/babel.js

@@ -6,7 +6,6 @@ const babelParserVersion = require("@babel/parser/package.json").version;
 exports.options = {
   allowImportExportEverywhere: true,
   allowReturnOutsideFunction: true,
-  // This won't work until @babel/parser@7.5.0 is released.
   allowUndeclaredExports: true,
   plugins: [
     "*", "flow", "jsx",