Remove support to django 1.6 and 1.7

[patrick91]

Closes #187 and it is helpful for #186 :)

[coveralls]

Coverage decreased (-3.04%) to 90.76% when pulling 575e1da on patrick91:feature/up-django into afec960 on graphql-python:master.

[coveralls]

Coverage decreased (-2.4%) to 91.398% when pulling 245ebe3 on patrick91:feature/up-django into afec960 on graphql-python:master.

[coveralls]

Coverage decreased (-1.7%) to 92.102% when pulling 82055ac on patrick91:feature/up-django into afec960 on graphql-python:master.

[coveralls]

Coverage decreased (-1.09%) to 92.71% when pulling 82055ac on patrick91:feature/up-django into afec960 on graphql-python:master.

[patrick91]

@syrusakbary can you have a look at this one as well? :)

Not sure why the coverage has decreased.

[syrusakbary]

I've analyzed the downloads for Django 1.6, 1.7 vs the newest ones in Google BigQuery database for PyPI:

SELECT
  MONTH(timestamp) as month,
  file.version,
  COUNT(*) as total_downloads
FROM
  TABLE_DATE_RANGE(
    [the-psf:pypi.downloads],
    TIMESTAMP("20170401"),
    CURRENT_TIMESTAMP()
  )
WHERE
  file.project = 'django'
GROUP BY
 month,
 file.version
ORDER BY
file.version,
month
LIMIT 1000

And the downloads for Django 1.6, Django 1.7 are now considerably low for the latest month compared to the most recent versions.

I think is reasonable to deprecate support for it (will add a small note in the README)

Merging :)