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:
  #33

pip_accel/__init__.py

@@ -23,7 +23,7 @@
 """
 
 # Semi-standard module versioning.
-__version__ = '0.14'
+__version__ = '0.14.1'
 
 # Standard library modules.
 import logging
@@ -75,7 +75,7 @@
 # The version number of the binary distribution cache format in use. When we
 # break backwards compatibility we bump this number so that pip-accel knows it
 # should clear the cache before proceeding.
-CACHE_FORMAT_REVISION = 5
+CACHE_FORMAT_REVISION = 6
 
 def main():
     """

pip_accel/utils.py

@@ -16,6 +16,7 @@
 # Standard library modules.
 import os
 import pipes
+import platform
 import pwd
 import re
 import sys
@@ -49,12 +50,25 @@ def expand_user(pathname):
 
 def get_python_version():
     """
-    Return a string identifying the currently running Python version.
+    Get a string identifying the currently running Python version.
 
-    :returns: A string like "py2.6" or "py2.7" containing a short mnemonic
-              prefix followed by the major and minor version numbers.
+    This function generates a string that uniquely identifies the currently
+    running Python implementation and version. The Python implementation is
+    discovered using :py:func:`platform.python_implementation()` and the major
+    and minor version numbers are extracted from :py:data:`sys.version_info`.
+
+    :returns: A string containing the name of the Python implementation
+              and the major and minor version numbers.
+
+    Example:
+
+    >>> from pip_accel.utils import get_python_version
+    >>> get_python_version()
+    'CPython-2.7'
     """
-    return "py%i.%i" % (sys.version_info[0], sys.version_info[1])
+    return '%s-%i.%i' % (platform.python_implementation(),
+                         sys.version_info[0],
+                         sys.version_info[1])
 
 def run(command, **params):
     """