chore: update GitHub branch references to use HEAD

CONTRIBUTING.md

@@ -56,7 +56,7 @@ We welcome direct contributions to the smtpapi-php code base. Thank you!
 
 ##### Prerequisites #####
 
-Please see [composer.json](https://github.com/sendgrid/smtpapi-php/blob/master/composer.json).
+Please see [composer.json](composer.json).
 
 ##### Initial setup: #####
 
@@ -81,7 +81,7 @@ source ./sendgrid.env
 
 ##### Execute: #####
 
-See the [examples folder](https://github.com/sendgrid/smtpapi-php/tree/master/examples) to get started quickly.
+See the [examples folder](examples) to get started quickly.
 
 <a name="understanding-the-codebase"></a>
 ## Understanding the Code Base
@@ -103,9 +103,9 @@ Source code.
 
 All PRs require passing tests before the PR will be reviewed.
 
-All test files are in the [`test`](https://github.com/sendgrid/smtpapi-php/tree/master/test) directory.
+All test files are in the [`test`](test) directory.
 
-For the purposes of contributing to this repo, please update the [`Header.php`](https://github.com/sendgrid/smtpapi-php/blob/master/test/Smtpapi/Header.php) file with unit tests as you modify the code.
+For the purposes of contributing to this repo, please update the [`Header.php`](test/Smtpapi/HeaderTest.php) file with unit tests as you modify the code.
 
 ```bash
 cd test
@@ -169,7 +169,7 @@ If you can, please look at open PRs and review them. Give feedback and help us m
 5. Locally merge (or rebase) the upstream development branch into your topic branch:
 
    ```bash
-   git pull [--rebase] upstream master
+   git pull [--rebase] upstream main
    ```
 
 6. Push your topic branch up to your fork:
@@ -179,7 +179,7 @@ If you can, please look at open PRs and review them. Give feedback and help us m
    ```
 
 7. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/)
-    with a clear title and description against the `master` branch. All tests must be passing before we will review the PR.
+    with a clear title and description against the `main` branch. All tests must be passing before we will review the PR.
 
 <a name="code-reviews"></a>
 ## Code Reviews

README.md

@@ -1,6 +1,6 @@
-![Twilio SendGrid Logo](https://github.com/sendgrid/sendgrid-python/raw/master/twilio_sendgrid_logo.png)
+![Twilio SendGrid Logo](twilio_sendgrid_logo.png)
 
-[![BuildStatus](https://api.travis-ci.org/sendgrid/smtpapi-php.png?branch=master)](https://travis-ci.org/sendgrid/smtpapi-php)
+[![BuildStatus](https://api.travis-ci.org/sendgrid/smtpapi-php.png?branch=main)](https://travis-ci.org/sendgrid/smtpapi-php)
 [![Latest Stable Version](https://poser.pugx.org/sendgrid/smtpapi/version.png)](https://packagist.org/packages/sendgrid/smtpapi)
 [![Latest Version on Packagist](https://img.shields.io/packagist/v/sendgrid/smtpapi.svg?style=flat-square)](https://packagist.org/packages/sendgrid/smtpapi )
 [![Email Notifications Badge](https://dx.sendgrid.com/badge/php)](https://dx.sendgrid.com/newsletter/php)
@@ -16,7 +16,7 @@ Learn more about the SMTP API at [Twilio SendGrid's documentation](https://sendg
 # Announcements
 **The default branch name for this repository has been changed to `main` as of 07/27/2020.**
 
-All updates to this module is documented in our [CHANGELOG](https://github.com/sendgrid/smtpapi-php/blob/master/CHANGELOG.md).
+All updates to this module is documented in our [CHANGELOG](CHANGELOG.md).
 
 # Table of Contents
 
@@ -97,7 +97,7 @@ print $header->jsonString();
 # Usage
 
 - [Twilio SendGrid Docs](https://sendgrid.com/docs/API_Reference/SMTP_API/index.html)
-- [Example Code](https://github.com/sendgrid/smtpapi-php/tree/master/examples)
+- [Example Code](examples)
 
 <a name="roadmap"></a>
 ## Roadmap
@@ -107,14 +107,14 @@ If you are interested in the future direction of this project, please take a loo
 <a name="how-to-contribute"></a>
 ## How to Contribute
 
-We encourage contribution to our projects, please see our [CONTRIBUTING](https://github.com/sendgrid/smtpapi-php/blob/master/CONTRIBUTING.md) guide for details.
+We encourage contribution to our projects, please see our [CONTRIBUTING](CONTRIBUTING.md) guide for details.
 
 Quick links:
 
-- [Feature Request](https://github.com/sendgrid/smtpapi-php/blob/master/CONTRIBUTING.md#feature_request)
-- [Bug Reports](https://github.com/sendgrid/smtpapi-php/blob/master/CONTRIBUTING.md#submit_a_bug_report)
-- [Improvements to the Codebase](https://github.com/sendgrid/smtpapi-php/blob/master/CONTRIBUTING.md#improvements_to_the_codebase)
-- [Review Pull Requests](https://github.com/sendgrid/smtpapi-php/blob/master/CONTRIBUTING.md#code-reviews)
+- [Feature Request](CONTRIBUTING.md#feature_request)
+- [Bug Reports](CONTRIBUTING.md#submit_a_bug_report)
+- [Improvements to the Codebase](CONTRIBUTING.md#improvements_to_the_codebase)
+- [Review Pull Requests](CONTRIBUTING.md#code-reviews)
 
 <a name="about"></a>
 # About