Fix Travis CI builds

[tterb]

Description

• Updated minimum PHP version from v5.4 to v5.6 because it's now the oldest version that is supported by WordPress
• Modified CI to use Trusty build environment because Xenial no longer supports PHP v5.x
• Updated dependency versions in composer.json and composer.lock to be compatible with Composer v2.0
• Added PHP v7.3 to the CI build versions
• Added dependency caching to improve build performance
• Added bin/test.sh to prevent separate CI builds being created for WP_MULTISITE
• Modified the phpunit command to use the version that is specified by Composer

Motivation and Context

The CI build was broken in every way possible.

How Has This Been Tested?

https://travis-ci.org/github/disqus/disqus-wordpress-plugin/builds/753276884

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

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This change is 

[tterb]

The only thing I'm not sure about is if we should update the PHP version in the deploy process from v7.0 to v7.3 since it looks like it was intentionally changed from v7.1 in the past.