Add version comparison utility

Source: http://stackoverflow.com/a/1714190/146925
Credit: gnud <http://stackoverflow.com/users/27204/gnud>
Support pre-5.20131213 versions of git-annex for EL6

Older versions of git-annex don't support `git annex lock --force`, so peform
a version check and call without --force on these.

The EL6 Docker image is changed to one that contains locales, in order for
git-annex to run successfully.