Add dep versioning and update tested Go versions

[mbrukman]

• add go.mod and go.sum files to manage dependencies with pinned versions for consistency and reproducibility
• add env var GO111MODULE=on to .travis.yml to ensure use of go mod for dependency versioning and management, which was introduced in Go 1.11 and was optional then, and automatic in later versions
• this functionality requires Go 1.11 or later, but as noted in issue #72, Go 1.12 and older are already technically EOL due to the release of 2 more major versions since then (1.13 and 1.14)
• update .travis.yml to remove all versions prior to Go 1.11; also added newer versions of Go: 1.12, 1.13, 1.14

Fixes #72

[mbrukman]

Looks like the Go 1.11 build failed because one of the transitive dependencies requires a minimum of Go 1.12:

go build golang.org/x/sys/unix: module requires Go 1.12

so I will update the PR accordingly and drop Go 1.11 from the list of tested versions, and will also update go.mod and the README.md to notify users of this, and note the need for GO111MODULE=on as well.

[mbrukman]

Thanks for review + approval, @robfig! The tests are passing; is there anything else you'd like to see before merging?

[robfig]

Nope, merging