Drop old python versions

[jasonpenny]

• A dependency of pytumblr dropped support for 2.6
  Drop support for Python 2.6 requests/requests-oauthlib#273

• Travis CI does not support 3.7 (with the default linux dist), but this library works with 3.7
  https://docs.travis-ci.com/user/languages/python/#development-releases-support

• 3.3 reached its end-of-life in September 2017 and the download timed-out in Travis CI
  https://travis-ci.org/tumblr/pytumblr/jobs/476949997
  https://www.python.org/downloads/release/python-337/

• increments pytumblr version for publishing to pypi

[hugovk]

• Travis CI does not support 3.7, but this library works with 3.7
  docs.travis-ci.com/user/languages/python/#development-releases-support

Travis CI does support 3.7, but it needs dist: xenial. So try something like this:

python:
  - 2.7
  - 3.4
  - 3.5
  - 3.6
# Enable 3.7 without globally enabling dist: xenial for other build jobs
matrix:
  include:
    - python: 3.7
      dist: xenial

Or this:

matrix:
  include:
    - python: 2.7
    - python: 3.4
    - python: 3.5
    - python: 3.6
    - python: 3.7
      dist: xenial

[jasonpenny]

Thanks @hugovk, that seemed to work

[hugovk]

Python 3.4 is also EOL in a couple of months (2019-03-16), you could consider dropping that too.

Here's the pip installs for PyTumblr from PyPI for December 2018:

category	percent	downloads
2.7	88.97%	10,231
3.6	5.51%	634
3.7	2.88%	331
3.5	1.95%	224
null	0.53%	61
3.4	0.15%	17
2.6	0.02%	2
Total		11,500

Source: pypistats python_minor --last-month PyTumblr