Switch from Travis to GitHub Actions

[mogren]

Minor PR to make cboring use GitHub Actions, just like dtn7-go.

Changes:

• Add Action running go build, go test and golangci-lint on code push and PRs
• Remove .travis.yml config file
• Add a .gitignore file
• Update BP draft link in Readme to revision 29
• Fix go fmt issues in test files

Sample Action run: https://github.com/mogren/cboring/actions/runs/371183684

[mogren]

@oxzi Thanks, I should have seen that the banner needed to be updated as well. Thanks for the suggestion, applied it.