Remove Python 2.6, add Python 3.6

[hugovk]

Resolves #304.

Edit: CI 3.3 failure didn't on my build: https://travis-ci.org/hugovk/thriftpy/builds/288487124 Is it a known, transient error?

[hugovk]

Here are pip installs from PyPI for the last month:

pypinfo --percent --pip thriftpy pyversion
python_version percent download_count
-------------- ------- --------------
2.7              71.9%         57,022
3.6              11.1%          8,821
3.4               8.6%          6,828
3.5               8.3%          6,554
2.6               0.1%            108
None              0.0%              8
3.3               0.0%              8
3.7               0.0%              4

Python 3.3 is also EOL, how about dropping that too?

[hugovk]

@julioasotodv @hit9 Hello, any thoughts on this PR? Thank you!

[julioasotodv]

Yeah looks good to me. Nobody in py3 uses 3.3 anymore

[hugovk]

PR updated to drop EOL Python 3.3.

(I get the same CI failure on latest develop: https://travis-ci.org/hugovk/thriftpy/builds/288453394)

[hugovk]

@hit9 Hi, what do you think about this PR? Thank you.