<feat>[!]: Use go module

[titouanfreville]

Content

Introduce go modules usage to manage versioning and dependencies.

I think it is a breaking change as it will require an upgrade in the release tag system and the import path will be upgraded to sendgrid-go/v3 while module name remain unchanged.

Concern #395. It does not fix it as is but it is strongly related to it.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them (At least I think so)
• I have titled the PR appropriately
• I have updated my branch with the master branch
• I have added tests that prove my fix is effective or that my feature works => I ensured that test are not broken by the migration
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified => added a go.mod file and changed import path according to it.

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[titouanfreville]

Passing to go module is also breaking import on go version under v1.9 (cf https://travis-ci.org/github/sendgrid/sendgrid-go/builds/694810575)
I think it can be fix by moving code for v3 under a v3 package path as exposed in https://blog.golang.org/v2-go-modules

Should I integrate it ?

[shwetha-manvinkurke]

Closing as a duplicate of #441