[CI] Test only the nearest two major go versions

[tyru]

On the latest go command appends go 1.13 at the bottom of go.mod.

https://travis-ci.org/vim-volt/volt/jobs/537348218

$ go version
go version devel +d97bd5d07a Sat May 25 22:38:01 2019 +0000 linux/amd64

This PR avoids that.

[tyru]

As of go 1.12, go command appends language version if missing (ref.: golang/go@48b7bdd0d7)

https://golang.org/doc/go1.12#modules

The go directive in a go.mod file now indicates the version of the language used by the files within that module. It will be set to the current release (go 1.12) if no existing version is present. If the go directive for a module specifies a version newer than the toolchain in use, the go command will attempt to build the packages regardless, and will note the mismatch only if that build fails.

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It is hard to track go tip, and Volt doesn't need to. it is enough to test only the nearest two major go versions