Add branch to support Yii 2.1

[robregonm]

What steps will reproduce the problem?

Create a new branch using Yii 2.1

What's expected?

It should work in composer (it works with some hacks)

What do you get instead?

Add a branch including Yii 2.1 as supported version in composer.json.

[cebe]

we'll add a branch when Yii 3.0 is at a stage that allows for it, no need to have an issue here.