Use HTTPS for Vimeo OAuth and endpoint requests.

Using HTTPS ensures, among other advantages, that URLs to assets returned by (e.g.) the videos endpoint will point to the version of the resource served over HTTPS, so that we get `https://secure-b.vimeocdn...` instead of `http://b.vimeocdn...`.

I was unable to check whether this breaks any specs, as I couldn't get the test suite to run -- #33.