Updated composer.json - Added Composer Installers "Type" Property

[simondowdles]

Added "type": "wordpress-plugin" to composer.json so that hosts may define a custom path for CMB installation

Resolves N/A

I have:

• [ x ] Run WordPress VIP PHPCS check and code parses without errors
• [ x ] Added any new PHPUnit tests
• [ x ] Run PHPUnit and all tests are passing after adding any necessary new tests
• [ x ] Tested feature/bugfix on single and multisite install

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A small change, but one that we have needed on a few occasions, and that is to specify where CMB is installed, especially when including it in the inc or library folder of another plugin.

[mikeselander]

Thanks for submitting @simondowdles - will take a look at this shortly