Fixes bug in compiler modified_time handling

I had the same issue as #211 (and possibly #210) and this seems to be a reasonable solution.  When I dug into the issue, I found:

* The compiler passes absolute file paths for infile and outfile when trying to determine modification times.
* The modified_time method on Django's file storage objects uses the storage's path() method as part of its determination
* path() expects a storage-relative name, which is at odds with passing absolute file paths
* path() doesn't find the named file, and falls back to the default StaticFilesStorage handling, which for some reason looks for the absolute file in the static directory of the first app in my INSTALLED_APPS, notices that the requested file is outside of that directory, and raises a SuspiciousOperation error.

FYI, I'm relying on the `django.contrib.staticfiles.finders.AppDirectoriesFinder' finder, which may be part of the cause of this issue.