Reflex now requires Go version v1.13 or higher

[okybr]

The lastest commit -- 13e5691 -- uses errors.Is (from the errors package) which was introduced with Go version v1.13. (You can compare the documentation of the different versions like https://pkg.go.dev/errors@go1.13.) But the README of this package still states that it only requires v1.11. Thus, there's a contradiction which can be resolved

• either by requiring at least Go version v1.13 and adjust the README; or
• modify reflex.go again so that it does not use errors.Is so that this package still works with Go v1.11.

As the service ran by my company does not yet use v1.13, we'd prefer you to choose the second way.

[cespare]

I'd rather keep using the new features. I updated the installation instructions in the README.

I've added binary downloads to the Releases page so you can install that way if you wish.