When updating devices with readings seen since last update, convert F to C

[phad]

Code in app/models/device_session.rb to handle client requests for new readings since last update didn't convert from F to C for users whose preference is C.

[hebnern]

Sweet, thanks! I'll get this deployed soon.

[phad]

I'm afraid this has introduced a new bug: when the new readings are added to the graph they are assumed to be fahrenheit, and go through a second conversion to celcius.
So the for celcius users the graph then dips down with the ajaxed-in readings.
A page refresh fixes it as then the whole graph is re-seeded with values from the readings file.
I didn't realise that temp conversion is also done in the inline JS in the _devices_session.html.erb file.
Looking at that now i think the correct fix would be to revert the change I made in device_session.rb and to add an <% if @device.user.temperature_scale == 'C' -%> section in the html.erb file to convert the most recent new reading to C just before updating the last-reading-n and last-setpoint-n DOM elements.

wdyt?

If you've not already deployed the previous fix can you hold off and I'll send a new PR with this different approach?

[phad]

New PR with the alternative proposed fix: #5