Drop support for python 2.6

[nichochar]

We currently support it, but it's in end of life, and a lot of major libraries are dropping support in the near future:

• pip
• setuptools

It also forces us to use an old version of gevent in our tests, because of the following error:

```Collecting gevent (from -r test-requirements.txt (line 4))
  Downloading gevent-1.2.1.tar.gz (2.8MB)
    100% |████████████████████████████████| 2.8MB 510kB/s 
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-rPyEfW/gevent/setup.py", line 39, in <module>
        raise Exception("Please install gevent 1.1 for Python 2.6")
    Exception: Please install gevent 1.1 for Python 2.6```

@jparise @cgordon any thoughts?

[jparise]

I agree, especially now that PyPy is 2.7 compatible. Years ago, that was a reason to maintain 2.6 support.

Let's make this change in a minor point release.

[nichochar]

Great, I think @jogo also agree, so that settles it, I'll make a PR and make a minor release after #137 and #138 go out.