Update dockerfile, drop python2.7

[jreiberkyle]

Update the Dockerfile for the following:

• drop python 2.7, focus support on python 3.6
• move label-maker installation to Dockerfile in label-data folder
• move dependencies into requirements.txt file (note, gdal conda install is still only in Dockerfile)

Update notebooks to run in python3.6.

Closes issues #29, #28, and #27

[sarasafavi]

Hooray, Python 3!