Upgrade firebase/php-jwt

[TomA-R]

What?

• Upgrade firebase/php-jwt
  • Support version ~6.0. This required a test update for JWT::decode, see https://github.com/firebase/php-jwt/releases/tag/v6.0.0
  • Remove support for version ~3.0. This version is very old and does not support the above test update
• Remove .travis.yml because we no longer use Travis
• Minor README updates

Why?

To address CVE-2021-46743

Testing

1. Tests still pass:
   

2. I manually verified this with the following script

use Bigcommerce\Api\Client as Bigcommerce;

require 'vendor/autoload.php';

Bigcommerce::configure([
    'client_id' => '...',
    'auth_token' => '...',
    'store_hash' => '...',
    'client_secret' => '...',
]);

var_dump(Bigcommerce::getCustomerLoginToken(1));

Which produced the expected result.

ping @bigcommerce/php