Use newer version of digi-xbee to fix performance issue

[aidanbh]

The digi-xbee library on pip was last released April 2018. This code leaks StreamHandlers which results in logging messages being duplicated. I believe this is the source of an issue which causes CPU usage to massively spike when a lot of packets are queued for transmit, creating a large interval between bursts of serial Tx/Rx during which the screens is filled with logged byte messages. A few weeks ago, digidotcom/xbee-python#57 was discussed at digi upstream which revealed that a patch (digidotcom/xbee-python@dad8aae) came out in August that fixed this, but one one ever uploaded to pip. So we'll need to pull from git.

When I get home today I'll update the Pipfile and test it out.

Since it is already quite wasteful to log every packet sent/recd (doubling xfer times, bc TUN and stdout together are just two file descriptors instead of one), I'm going to go ahead and turn it off entirely. It would be nice if they had a way to disable their logs while still letting me use whatever log levels I want in my code, but their response (digidotcom/xbee-python#61) was to call logging.disable(). I'll dig around and see if there's another way.

[aidanbh]

Closed by 5065ef8. Also fixes a different leak in the library patched right after!