Drop support for Python 2.6

I would humbly like to suggest isort drops support for Python 2.6.

The last release of Python 2.6 was 2013-10-29, over 3 years ago. It is
no longer receiving security fixes.

https://www.python.org/dev/peps/pep-0361/

The pip project itself has recently dropped support for 2.6. Their
numbers estimate that Python 2.6 accounts for ~2% of their downloads.
See:

https://github.com/pypa/pip/pull/4343

For projects that still use Python 2.6, they can continue to pip install
an older version of isort.

I've tried my best to remove as much 2.6 specific code as I can,
including the 'Programming Language :: Python :: 2.6' trove classifier
from setup.py. I've also removed Travis CI testing, which should result
in faster testing and fewer wasted resources.

Code changed:

- Removed Python2.6 from documentation
- Removed Python2.6 from testing configuration
- Use set literals
- Use dict comprehension
- Pass generator expressions to builtin functions instead of lists
- Use builtin OrderedDict
- Removed deprecated U open mode; replace with io.open() which support
  universal newlines
- Removed Python2.6 workarounds throughout the code

Thanks for considering.