Add build checks for go fmt and go mod tidy

[markfuller]

Is your feature request related to a problem? Please describe.
Add some build checks to ensure that go mod tidy and go fmt ./... would not make any changes.

Describe the solution you'd like
In travis:

• If go fmt ./... changes any files, fail the build
• If go mod tidy changes go.sum, fail the build

Describe alternatives you've considered
N/A

Additional context
This will require an upgrade to go 1.12 due to golang/go#27868 (contributors could work locally without it but any PRs containing modifications to go.sum are likely to be rejected)

Travis (go stable) is already running 1.12.5.