Encode Python implementation in binary cache filenames

While testing the results of merging pull request #33 I noticed that
when I enabled the Amazon S3 cache backend and ran the test suite using
Tox (which runs the test suite under CPython 2.6, CPython 2.7, CPython
3.4 and PyPy 2.7) either the CPython 2.7 or the PyPy 2.7 tests would
fail. It turns out that the two don't go well together (who would have
thought? ;-). This change set permanently fixes the problem by encoding
the Python implementation in the filenames used to store binary
distribution archives in the binary cache.

See also pull request #33 on GitHub:
  https://github.com/paylogic/pip-accel/pull/33