requirements.txt: change package name "hub" to "git-spindle"

Package "hub" has been renamed some time ago to "git-spindle".
Recently package with the same name "hub" has been released on PyPi
page, which causes installing pip requirements.txt to fail since
it is trying to install different package.

Signed-off-by: Marcin Sloniewski <marcin.sloniewski@gmail.com>