Define and implement Go version policy

[MichaelRybak]

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