fixed cache invalidation when file modified in Google Drive

Fixes issue #4

I noticed that when the file objects have a `lastModifiedDate` property when they are read from Google Drive but not the local filesystem. Also the date is automatically updated even though the file is not, that is, reading the file still returns the old contents. So the solution seems pretty simple: save the date along with the file and then check it when reading from the cache, and invalidate the cache if they don't match.