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.

fisher.fish

@@ -817,8 +817,10 @@ function __fisher_remote_index_update
     end
 
     if test -s "$index"
-        if test (__fisher_get_file_age "$index") -lt "$interval"
-            return
+        if set -l file_age (__fisher_get_file_age "$index")
+            if test "$file_age" -lt "$interval"
+                return
+            end
         end
     end