Updating git minimum version to 1.8.0

#80 outlines an issue where git push doesn't have the --prune option below git 1.8.0.

Author: samrocketman

docs/prerequisites.md

@@ -5,7 +5,7 @@
 * [GitLab 6.x/7.x][gitlab]
 * [pyapi-gitlab3 @ v0.5.4][python-gitlab3]
 * [GNU coreutils][coreutils]
-* [git 1.6.5][git] or later (git 1.6.5 introduced transport helpers)
+* [git 1.8.0][git] or later
 
 If you plan on mirroring SVN repositories as well then you'll need the
 following additional options.