check modified date of config file when caching

Right now we don't re-load a config file if it is cached.
This is a bug because the file could change while the code is running and the cache will not be cleared.
We should also keep track of the last modified date and only if it is before the cached date use the file.

ref: https://stackoverflow.com/questions/375154/how-do-i-get-the-time-a-file-was-last-modified-in-python