Add composer validate job in Travis.

[connorshea]

Description

This adds a job to Travis that validates that the composer.json and composer.lock are in sync.

Motivation and Context

I forgot to update the composer.lock when adding php-cs-fixer and had to fix it in #7450. This job shouldn't take very long to run and would fix this problem.

How To Test This

Make sure Travis passes.

Types of changes

• 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)

Final checklist

• My code follows the code style of this project found here.
• My change requires a change to the documentation.
• I have read the How to Contribute guidelines.

[Mac-Rae]

Assessed by Travis 👍