Replace ':' char for windows-safe filenames

The cache key cannot have ':' chars because this is not allowed
in filenames for windows

(http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx)

This means that the assume role functionality does not work on windows
until this issue is fixed.
Create cache file with 0600 permissions
Add issue to changelog
Merge branch 'windows-cache-names' into develop

* windows-cache-names:
  Add issue to changelog
  Create cache file with 0600 permissions
  Replace ':' char for windows-safe filenames