upgrade to @babel/generator@7.5.0

Summary:
This version is needed to correctly generate inexact flow object type
annotations.

Keeping the upgrade minimal to reduce exposure to runtime code generation and avoiding a full babel upgrade.

Changelog: [Internal]

Reviewed By: jbrown215

Differential Revision: D18429751

fbshipit-source-id: d83621ed773566005a4dcdd4f99ded5101dc74b4