Support go modules - Githubissues

To make it easier and more consistent for other libraries and applications to use the pcidb library, we should support 1.11+ go modules.

[x] Add a go.mod and go.sum file containing pcidb's dependencies
[x] Use proper semver three-tuple release style since go modules apparently don't understand X.Y without a X.Y.Z component
[x] Add travis gimme environments for testing a non-$GOPATH install using GO111MODULE=on

jaypipes / pcidb