Remove Python 2.6 from the build in .travis.yml

[rudaporto]

Python 2.6 is no longer supported by sqlalchemy>=1.2 since is not supported anymore by the python community.

Can we just remove it from .travis.yml and also clean up all the code that supports python 2.6?

This is also blocking this pull request since the tests are not passing in Python 2.6:

• Avoiding full table scan when looking for inclusions. #677

[hugovk]

Python 2.6 has been EOL for over 4 years and is no longer receiving security updates from the core Python team, so it's about time to drop it to reduce maintenance burden and allow newer features of Python to be used.

It's also little used. Here's the pip installs for Flask-Restless from PyPI for last month:

python_version	percent	download_count
2.7	82.5%	7,509
3.6	9.1%	830
3.5	4.7%	428
3.4	3.0%	275
3.3	0.3%	29
2.6	0.3%	25
3.7	0.1%	7

Source: pypinfo --start-date -57 --end-date -27 --percent --pip --markdown Flask-Restless pyversion

Python 3.3 is also EOL and little used, that could be dropped at the same time.