Fix Python 2.6 support

The latest version of IPython, 2.0.0, dropped Python 2.6 support.
We fix this by splitting up the test requirements into the usual
requirements-test.txt, and requirements-test-py26.txt,
which is Python 2.6 specific. Both .travis.yml and tox.ini
are updated to use this new file.

Other changes:
- Removed apt-get install libzmq in .travis.yml
  since we don't use it anymore.