Removed `_in_stack` since it was slow.
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.