Composer: update dev dependencies

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• Update dev dependencies

Relevant technical choices:

Update various dev dependencies:

• PHP Parallel Lint from v 1.2.0 to 1.3.0
• PHPCompatibilityWP from 2.1.0 to 2.1.1
• PHPCompatibilityParagonie from 1.3.0 to 1.3.1
• PHP_CodeSniffer from 3.5.8 to 3.6.0
• Mockery from 1.3.3 to 1.3.4

Refs:

• https://github.com/php-parallel-lint/PHP-Parallel-Lint/releases/tag/v1.3.0
• https://github.com/PHPCompatibility/PHPCompatibilityWP/releases/tag/2.1.1
• https://github.com/PHPCompatibility/PHPCompatibilityParagonie/releases/tag/1.3.1
• https://github.com/squizlabs/PHP_CodeSniffer/releases/tag/3.6.0
• https://github.com/mockery/mockery/releases/tag/1.3.4

Test instructions

This PR can be tested by following these steps:

• N/A If the build passes, we're good.