pebble-sdk 2.8.1

Bump pebble-sdk to version 2.8.1.

Also updated pyserial requirement to the latest compatible version, 2.7.

Closes #34887.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
pebble-sdk: update 2.8.1 bottle.
Upgrade pyenv to v20141211
pyenv: add 20141211 bottle.