Bump version to 1.3.0

[jeffreyparker]

Summary

• Version bump for the 1.3.0 release containing all CA pinning improvements
• Includes: disable_ca_pinning option (Add an option to disable CA certificate pinning #51), User-Agent reporting (Add CA bundle version and pinning status to the user agent string #52), appendToUserAgent() (Add appendToUserAgent() method to Client class #53), CURLOPT_CAPATH fix (Prevent OS certificate store fallback when CA pinning is enabled #54)

Test plan

• vendor/bin/phpunit tests/ — all 83 tests pass
• After merge: tag 1.3.0, create GitHub release, verify Packagist sync