switch to jessie and drop dist-upgrade

as recommended by the Docker team in our PR to `official-images`.

@eddelbuettel look okay?  I suppose tying this to Jessie means that when the next Debian release is named we have to update things (e.g. have separate directories corresponding to the version tags for each of the different releases).  Guess we can cross that bridge when we get there.  

I also added an inline note about setting locales, linking to issue #19 which discussed that choice. If this looks good then go ahead and merge and then I'll update the PR to official images, adding the tag for 3.1.1 and commenting on the things we've addressed.