Minor refactoring after handling File.expand_path on BOOKMARKS_FILE
Changed File method from atime to mtime

The atime method returns the file's last access time, whereas the mtime
method returns the modification time. Both methods returns the time as a
Time object.

Since we are checking for the most recent time a bookmark was added,
updated or deleted on Pinboard.in and comparing this to our local file,
we need to make sure we are comparing the last time we actually wrote to
the file and not the last time the file was accessed.