Update .gitattributes to exclude development files

[BrianHenryIE]

Exclude specific development files from prefer-dist installs.

Description

Many of the files included in composer install easypost/easypost-php are only relevant to the project's own development, not the use of the package.

This PR updates .gitattributes to exclude unnecessary files.

Composer's --prefer-source option (and equivalent composer.json config) can be used to pull in the entire repository as necessary.

Testing

Not applicable.

Pull Request Type

Please select the option(s) that are relevant to this PR.

• 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 not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)