Walmart Inc. is pouring more firepower into its fledgling financial venture, scoring a $2.5 billion valuation for the startup and signaling its ambition to wade deeper into financial services.The world’s largest retailer is leading a funding round of more than $300 million alongside investment firm Ribbit Capital, according to people with knowledge of the matter. That marks a fresh valuation for the firm called One, majority-owned by the retail giant, which has been offering products targeting W...