* Check that the wheel cache directory is writable before we attempt to write
  cached files to them.

* Move the pip version check until *after* any installs have been performed,
  thus removing the extraenous warning when upgrading pip.

* Added debug logging when using a cached wheel.

* Respect platlib by default on platforms that have it separated from purlib.

* Upgrade packaging to 15.3.

  * Normalize post-release spellings for rev/r prefixes.

* Upgrade distlib to 0.2.1.

  * Updated launchers to decode shebangs using UTF-8. This allows non-ASCII
    pathnames to be correctly handled.

  * Ensured that the executable written to shebangs is normcased.

  * Changed ScriptMaker to work better under Jython.

* Upgrade ipaddress to 1.0.13.