fix: Proper-case primitive from DocuSign API causes fatal error

[trickeyone]

Proper-case primitive returned from DocuSign API causes fatal error

Fatal Error (1): Class 'Number' not found in [/path/to/app/vendor/docusign/esign-client/src/ObjectSerializer.php, line 274]

Replaces #41. Branched from current master with only type changes.

[trickeyone]

Looks like the checks are failing due to an outdated version of PHPUnit.

PHP Deprecated:  The each() function is deprecated. This message will be suppressed on further calls in /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/Util/Getopt.php on line 38
PHP Stack trace:
PHP   1. {main}() /path/to/project//docusign-php-client/vendor/bin/phpunit:0
PHP   2. include() /path/to/project//docusign-php-client/vendor/bin/phpunit:115
PHP   3. PHPUnit_TextUI_Command::main() phpvfscomposer:///path/to/project//docusign-php-client/vendor/phpunit/phpunit/phpunit:51
PHP   4. PHPUnit_TextUI_Command->run() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/TextUI/Command.php:100
PHP   5. PHPUnit_TextUI_Command->handleArguments() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/TextUI/Command.php:111
PHP   6. PHPUnit_Util_Getopt::getopt() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/TextUI/Command.php:240
PHP   7. each() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/Util/Getopt.php:38

Deprecated: The each() function is deprecated. This message will be suppressed on further calls in /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/Util/Getopt.php on line 38

Call Stack:
    0.0010     412656   1. {main}() /path/to/project//docusign-php-client/vendor/bin/phpunit:0
    0.0036     419632   2. include('phpvfscomposer:///path/to/project//docusign-php-client/vendor/phpunit/phpunit/phpunit') /path/to/project//docusign-php-client/vendor/bin/phpunit:115
    0.0121     899416   3. PHPUnit_TextUI_Command::main() phpvfscomposer:///path/to/project//docusign-php-client/vendor/phpunit/phpunit/phpunit:51
    0.0121     899528   4. PHPUnit_TextUI_Command->run() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/TextUI/Command.php:100
    0.0121     899528   5. PHPUnit_TextUI_Command->handleArguments() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/TextUI/Command.php:111
    0.0127     925752   6. PHPUnit_Util_Getopt::getopt() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/TextUI/Command.php:240
    0.0127     926192   7. each() /path/to/project//docusign-php-client/vendor/phpunit/phpunit/src/Util/Getopt.php:38

PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

[trickeyone]

I have a fix available that upgrades to PHPUnit 9 with minimal changes. If desired, I can push this commit here as well.

[trickeyone]

Tests are now failing because the Travis CI job is missing environment variables that are only configured for the main master branch's build. Is there a way to configure these so the check can be properly run?