* Add golangci-lint as linting tool

* Disable failing staticchecks to start; GitHub issue to handle coming soon

* Run `goimports -w` to repair all source files that have improperly
formatted imports

* makefile: Add ci-lint target to run on travis

This change adds a new make target for running golangci-lint on newly
added Go files only. This target is expected to run during Packer ci builds.

* .github/contributing: Add code linting instructions

* travis: Update job configuration to run parallel builds