Release on CRAN

[nuest]

Prerequisites

• stevedore is on CRAN
• [ ] sysreqs is on CRAN ISSUE: CRAN bound? r-hub/sysreqs#5
• update implementation so that it conditionally relies sysreqs

Steps

• document release process in README, cf. https://github.com/52North/sos4R/blob/master/DEV-README.md#development-versions-and-branches
• --as-cran option and vignette building/checking is activated in CI tests
• rhub::check_for_cran() succeeds
• re-activate full build matrix in .travis.yml
• get listed in https://cran.r-project.org/web/views/ReproducibleResearch.html
• add badges > https://github.com/ropensci/cchecksapi
• CI works for devel
  • https://ci.appveyor.com/project/nuest/containerit/build/job/w2v2h7tn5nn0sgwe/artifacts
• update existing badge linking to this issue, and add links based on https://www.r-pkg.org/pkg/containerit

[nuest]

Considering the long wait with sysreqs, it might be best to make those features optional via require("sysreqs"), see r-hub/sysreqs#5