Check for Git version

Zenflow requires git >= 1.8, but does not do any checks for this, and the default version of git that ships with Mac OS X is 1.7.x It would be nice to check for this and throw errors and exit early when an old git version is installed.

One of the commands which did not exist in earlier versions of git that is used by Zenflow is "git branch --set-upstream-to".
