Add go directive in go.mod

[sergefdrv]

In order to be more clear about supported version of Go, as well as to automate checking of language features used, go.mod file should include a go directive. As described in go command documentation:

The expected language version, set by the go directive, determines which language features are available when compiling the module. Language features available in that version will be available for use. Language features removed in earlier versions, or added in later versions, will not be available.

Originally posted by @sergefdrv in #148