Stop forcing Python 2.6 compatibility

Python 2.6 is not supported officially more than year already.
This commit removes Python 2.6 environments from 'Travis' and 'tox'
settings. It **does not** make PyDSTool unusable on Python 2.6. It just
stops forcing new changes to be Python 2.6 compatible.
Merge pull request #73 from z2v/feature/drop-python-2.6

Stop forcing Python 2.6 compatibility