Update gcalcli image to python3 (#504)
Update gcalcli image to python3 (#504)

The latest gcalcli uses shutil.get_terminal_size which isn't
backported to python2's shutil.
michael-k/dockerfiles-jfrazelle,ricardorsierra/dockerfiles,jessfraz/dockerfiles,ricardorsierra/dockerfiles,ricardorsierra/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,michael-k/dockerfiles-jfrazelle,jfrazelle/dockerfiles,michael-k/dockerfiles-jfrazelle,michael-k/dockerfiles-jfrazelle,ricardorsierra/dockerfiles,jfrazelle/dockerfiles,ricardorsierra/dockerfiles,michael-k/dockerfiles-jfrazelle