Switch secret storage from keyring to .Renviron

[aoles]

Abandon the notorious keyring package which is causing trouble under certain architectures and deployment environments such as shinyapps.io.

The idea is to store the key as plain environment variable, preferable in .Renviron. In order to ease-up the set-up procedure and to retain previous functionality introduce an intelligent way of updating .Renviron through a call to ors_api_key(<YOUR_API_KEY>). See ?Startup for R startup details.

To remain CRAN compliant, ask for user permission before updating .Renviron (see e.g. rsam).

[aoles]

Builds are failing on Travis CI because libsodium-dev system package on which keyring now indirectly depends is not available on Ubuntu 14.04.