Rename --version to --pipeline-version

Fixes an issue where ``aws datapipeline get-pipeline-definition``
exposed a ``--version`` argument which was shadowed by the top level
``--version`` argument.  This is similar to what was done for
the ``--query/--objects-query`` argument.
Merge branch 'datapipeline-rename' into develop

* datapipeline-rename:
  Rename --version to --pipeline-version
Update changelog with latest features