Bump version to 2.6.0

[dbgrandi]

Description

While SPM support was added back in #32, it did not include a definition
for the YMOverrideTestSupport library. This is pretty useful for those
who like to add tests around the state of feature flags.

@cshaines updated the SPM manifest in #40, but we never published a new
release. Bump the version to 2.6.0, then we can tag and publish to both
CocoaPods and SPM.

How Has This Been Tested?

This has been in use via a fork (https://github.com/cshaines/Override/) for
a while and was contributed back to this repo in #40. Just need to bump
the versions and publish it.

Screenshots (if appropriate):

N/A

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)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

License

I confirm that this contribution is made under an MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[stury]

Thanks @dbgrandi !