Add ruby 2.7.1 to travis

- Add ruby 2.7.1 to travis

- update change log
- bump to 3.1.0

perform_send_request setting for testing purposes

feature/settings_perform_send_request (#78)

* - add new mailer settings :perform_send_request
- when set to false, it averts the sendgrid api request
- in combination with return_response=true it will return the full mail object which will respond to to_json

* Version Bump, update readme and changelog

* - ASM.new requires symbols

Validate error response body for empty string

v3.0.1

Validate error response body for empty string (#75)

Mail 2.7 Refactor

Require Mail 2.7 in order to take advantage of Mail .unparsed_value, which greatly simplifies the code and reduces the potential for bugs and edge cases.

Dynamic Templates and Personalizations

• Update sendgrid-ruby. 6.0 (#51)
• dynamic templates
• dynamic api_key
• personalizations
• bug fixes

v0.2.1: Merge pull request #20 from tricknotes/update-ruby-versions

Update Ruby versions on Travis CI