Drop support for EOL Python 2.6

[hugovk]

Fixes #587.

Travis CI is failing because SQLAlchemy no longer supports Python 2.6, as it's been EOL for over four years:

Collecting SQLAlchemy>=0.8.0 (from Flask-SQLAlchemy==2.3.2.dev20180120) 
Downloading SQLAlchemy-1.2.1.tar.gz (5.5MB)
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-cHubfe/SQLAlchemy/setup.py", line 16, in <module>
        raise Exception("SQLAlchemy requires Python 2.7 or higher.")
    Exception: SQLAlchemy requires Python 2.7 or higher.

For example: PR #587.

Re: http://docs.sqlalchemy.org/en/latest/changelog/migration_12.html#targeting-python-2-7-and-up
and: zzzeek/sqlalchemy@1da9d37

Here's the pip installs for Flask-SQLAlchemy from PyPI for last month:

python_version	percent	download_count
2.7	85.8%	645,538
3.6	7.7%	57,602
3.5	3.8%	28,411
3.4	2.5%	18,775
2.6	0.1%	868
3.7	0.1%	532
3.3	0.0%	346
None	0.0%	4
3.2	0.0%	1

Source: pypinfo --start-date -51 --end-date -21 --percent --pip --markdown Flask-SQLAlchemy pyversion

Python 3.3 is also EOL and even less used, but not causing any problems yet. Should that be dropped too?