Use local pytest-cache loader

Additional code is required to properly initalize pytest-cache when it's installed into a local egg in the current working directory.

Fix from here:

    https://bitbucket.org/hpk42/pytest-cache/issue/8/pytest-cache-plugin-not-loading-when-using

Without this, I get:
.....
running build_ext
usage: setup.py [options] [file_or_dir] [file_or_dir] [...]
setup.py: error: unrecognized arguments: --clearcache