travis-ci support

[#62851348]
Issue pull request for branch travis-ci-62851348
Merge pull request #84 from softcover/travis-ci-62851348

Travis-CI support
added coveralls
README update
Exit with error message when publishing unbuilt Markdown books

[Delivers #63117862]
Issue pull request for branch 63117862-softcover-publish

[Delivers #63117862](https://www.pivotaltracker.com/story/show/63117862)
Merge branch 'stable'
added standin testa
marketing.yml include contact_email / hide_custom_domain_footer
Merge branch 'master' of https://github.com/softcover/softcover
Merge branch 'master' into contact-email
Merge pull request #87 from softcover/contact-email

Contact email in marketing.yml
Merge pull request #86 from softcover/63117862-softcover-publish

63117862 softcover publish
Merge branch 'master' into coveralls
silence coveralls output for local
Merge pull request #85 from softcover/coveralls

Added Coveralls!
Bump version number