Fix All the Things and All the Types

This release introduces several updates to our TypeScript and FlowTyped definitions. TypeScript users should update their package.json to v15.1.1.

Full Changelog

Changes:

• Merged #280 Update index.d.ts by @opavader
• Merged #281 Replace {} with any by @NeoLegends
• Merged #282 Made optional arguments for browser router optional in flow type by @gertsonderby, which fixed #268