Fixing the battery level for firmware 3.1.8

[bepitulaz]

I'm testing your code in firmware 3.1.8 and the value for battery level is very different with the value in Flower Care app. The value from the script is 39% and the value in app is 100%. So, I change the unpack format to <B6s and now the value is similar.

I also add the Gattlib dependencies in Readme, because when I followed your instruction the pip gave me error "can't found glib2.0". By the way, I'm in Ubuntu 16.04.

Note: This code is untested for the firmware < 3.1.8