Run file age check, only if we can get a value from __fisher_get_file_age.
Run file age check, only if we can get a value from __fisher_get_file_age.

This probably means the user does not have python installed on their
system and we will be download the index every time the user runs
ls-remote.

A more sensitive, python-less solution to check file age since the last
time the file was modified should be more desirable.

fisherman/fisherman