Put coverage testing back

[alexflint]

#110 broke test coverage using coveralls. We should get that working under github actions.

The .travis.yml file was:

language: go
go:
  - "1.12"
  - "1.13"
before_install:
  - go get github.com/axw/gocov/gocov
  - go get github.com/mattn/goveralls
  - if ! go get github.com/golang/tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
script:
  - $HOME/gopath/bin/goveralls -service=travis-ci
  - bash test/compile_with_go110.sh
  - bash test/compile_with_go111.sh
  - bash test/compile_with_go111_inside_gopath.sh

The coveralls badge in the README was like this:

-[![Coverage Status](https://coveralls.io/repos/alexflint/go-arg/badge.svg?branch=master&service=github)](https://coveralls.io/github/alexflint/go-arg?branch=master)