Default to the "oob" flow if httpuv isn't installed.

This modifies the logic in oauth token fetching to default to using the `oob`
flow (i.e. provide a URL and ask the user to copy-paste a code) whenever
`httpuv` isn't available to use locally.

PTAL @hadley @deflaux
I'd print a message here: `message("httpuv not installed, defaulting to out-of-band auth")` (and then also check that `use_oob` isn't already true

(Also `<-` please!)