Drop support for EOL Python 2.6, 3.3 and 3.4

[hugovk]

Fixes #80.

Here's the pip installs for eve-swagger from PyPI for March 2019:

category	percent	downloads
2.7	54.76%	1,122
3.6	29.53%	605
3.7	6.20%	127
null	3.61%	74
3.4	3.17%	65
3.5	2.73%	56
Total		2,049

Source: pypistats python_minor eve-swagger --last-month # pip install pypistats

[nicolaiarocci]

thanks!