Drop support for Python 2.6

[tysonclugg]

Python 2.6 hasn't been supported for a long time. There was an issue where coveralls failed to run causing the build to fail prompting this PR.

[tysonclugg]

That's an odd error from Travis...

Submitting coverage to coveralls.io...
Coverage submitted!
Couldn't find a repository matching this job.
'url'
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/coveralls/cli.py", line 75, in main
    log.info(result['url'])
KeyError: 'url'

[coveralls]

Coverage remained the same at 96.532% when pulling 1188cdb on tysonclugg:issue/drop-python-26 into 18acc1b on campaignmonitor:master.

[coveralls]

Coverage remained the same at 96.532% when pulling 1188cdb on tysonclugg:issue/drop-python-26 into 18acc1b on campaignmonitor:master.

[coveralls]

Coverage remained the same at 96.532% when pulling 1188cdb on tysonclugg:issue/drop-python-26 into 18acc1b on campaignmonitor:master.

[coveralls]

Coverage remained the same at 96.532% when pulling 1188cdb on tysonclugg:issue/drop-python-26 into 18acc1b on campaignmonitor:master.

[coveralls]

Coverage remained the same at 96.532% when pulling 1188cdb on tysonclugg:issue/drop-python-26 into 18acc1b on campaignmonitor:master.