Version Bump v5.1.1: sendgrid#307, fixes sendgrid#276 phpdoc and styl…

…e fixes

Version Bump v5.1.0: sendgrid#295 Upgrade sendgrid/php-http-client

Version Bump v5.0.9: Pull sendgrid#289 JsonSerializable

Version Bump 5.0.8: Add TOC to README, Add USE_CASES.md

Version Bump v5.0.7: Added Troubleshooting section

Version Bump v5.0.6: README documentation improvements

Version Bump v5.0.5: Update docs, unit tests and examples to include …

…Sender ID

Version Bump v5.0.4: Tests now mocked automatically against prism

Version Bump v5.0.3 content update

Version Bump v5.0.2: swagger/oai updates + accept header fix