Fix import version bug.

Since we are using "iris" as namespace package, its
__init__.py should not contain any other code than
declare_namespace(). So getting version string from
pkg_resources lib.

Document from setuptools:

  http://pythonhosted.org//setuptools/setuptools.html#namespace-packages

  You must NOT include any other code and data in a namespace
  package’s __init__.py. Even though it may appear to work during
  development, or when projects are installed as .egg files, it
  will not work when the projects are installed using "system"
  packaging tools – in such cases the __init__.py files will not
  be installed, let alone executed.

Change-Id: I7bf96d2876edeac725c0049a226694d69075ab26