Drop Python 2.6 support

[k-nut]

It was announced to the mailing list by @amercader in January that support for python 2.6 was going to be dropped with ckan 2.6 (what a wonderful coincidence).

https://lists.okfn.org/pipermail/ckan-dev/2016-January/009608.html

Starting from the next CKAN 2.6 version, the minimum Python version
required for CKAN will be Python 2.7 (we recommend using the latest
Python patch release).

There still are some backports and workaraounds for 2.6 in the code and the documentation needs upgrading.

Grep for 2.6

Python files

ckan/lib/util.py
13:# subprocess.check_output() isn't in Python 2.6.
15:# Python 2.6.

ckan/model/extension.py
17:        "Replaces stdlib operator.methodcaller in python <2.6"

ckan/tests/helpers.py
36:    # Python 2.6 doesn't have these, so define them here

ckanext/datapusher/tests/test.py
87:        # Httpretty crashes with Solr on Python 2.6,

ckanext/resourceproxy/tests/test_proxy.py
63:        # Httpretty crashes with Solr on Python 2.6,

Documentation:

doc/contributing/python.rst
77:We use `the Python standard library's logging module <http://docs.python.org/2.6/library/logging.html>`_

doc/contributing/string-i18n.rst
339:  Worse (doesn't work in Python 2.6):

doc/maintaining/installing/install-from-source.rst
36:Python                 `The Python programming language, v2.6 or 2.7 <http://www.python.org/getit/>`_

I'll prepare a PR to remove all of those things.