Define and implement Go version policy

Let’s establish a policy that determines the versions of Go amppackager is compatible with. Specifically:
* Which major releases: all of Go 1.10+, or some of them?
* Which minor releases: all of Go 1.10+.*, or some of them?
* When is the policy updated?

The policy must be reflected by the project’s processes and documentation:
* Travis must check against all compatible Go versions.
* Manual setup procedure in README.md should mention supported versions and capture the differences between the setup procedures - e.g. the need for “-mod=vendor” flag.
* Contributing Guidelines should mention the policy and how it’s enforced by Travis.
* Document the policy