refactor: use string map

cmd/semantic-release/main.go

pkg/condition/condition.go

pkg/condition/defaultci/default.go

pkg/condition/github/github.go

pkg/condition/github/github_test.go

pkg/condition/gitlab/gitlab.go

pkg/condition/gitlab/gitlab_test.go

pkg/condition/travis/travis.go

pkg/condition/travis/travis_test.go