Mention new (py26) ordereddict dependency in changelog and docs

Author: wence-

changelog/920.bugfix

@@ -1 +1 @@
-Fix non-determinism in order of fixture collection.
+Fix non-determinism in order of fixture collection.  Adds new dependency (ordereddict) for Python 2.6.

doc/en/getting-started.rst

@@ -9,7 +9,8 @@ Installation and Getting Started
 
 **dependencies**: `py <http://pypi.python.org/pypi/py>`_,
 `colorama (Windows) <http://pypi.python.org/pypi/colorama>`_,
-`argparse (py26) <http://pypi.python.org/pypi/argparse>`_.
+`argparse (py26) <http://pypi.python.org/pypi/argparse>`_,
+`ordereddict (py26) <http://pypi.python.org/pypi/ordereddict>`_.
 
 **documentation as PDF**: `download latest <https://media.readthedocs.org/pdf/pytest/latest/pytest.pdf>`_