Move phpstan/phpstan to require-dev

[martinssipenko]

Description

Move phpstan/phpstan to require-dev so that it's not installed in projects using this package.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• 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)

Checklist:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

[martinssipenko]

@Crell I'd really appreciate a release of this change as currently when updating to the latest version of this package, PHPSTan is installed when it should really not be. I caught this because the project where I use this package still uses an older version of PHPStan (0.12.x) and is now conflicting with 1.4.0 of this package.

[Crell]

Hm, good call. That was probably just sloppiness on my part, sorry. Though where is the extra plugin coming from? Once that's resolved I'm good with merging.

[martinssipenko]

CI is failing on 7.4 but I don't believe it's caused by my changes.

[Crell]

Kicked CI, works now. No idea what happened there.

Merged. Thanks! I'll tag a new release sometime today; I may as well see if there's any other small cleanup to do while I'm here.