Proposal: Remove Python 2.7.6 support

[Roach]

Description

PR #51 has tests failing because Travis's test runner errors out on 2.7..6: https://travis-ci.org/slackapi/python-slack-events-api/jobs/529462763

I submitted a PR to simply remove the 2.7..6 environment from the travis configs, but closed it when I remembered that we have specific request signing logic for Python 2.7..6:
https://github.com/slackapi/python-slack-events-api/blob/master/slackeventsapi/server.py#L47-L83

2.7..6 was released in 2013, and PSF actively recommends 2.7.15 for new downloads: https://www.python.org/download/releases/2.7/

I'd like to remove the custom logic and test config for 2.7.6.

[vvatelot]

Same for all current PR...
All failing with 2.7.6 python version, except for a PR that passed the tests 12 months ago.

[vvatelot]

#59

[alperkal]

Any decisions here?

[seratch]

👋 I think there is no objection here. I will move this forward and check the PRs blocked by this.