test travis on py2.6 too

I noticed today that since merging #670 (two weeks ago) our master build is broken on Python 2.6 (which we did not discontinue -- not that I am aware of, anyway).

`obspy-runtests` in the 2 py 2.6 debuntu buildbots was erroring before it was getting to the testing so the respective (failing) tests simply did not appear on `tests.obspy.org`.

This PR makes Travis test Python 2.6, too.

However, we can not install binary packages via `apt-get` for 2.6 and have to build dependencies via `pip install`. Compiling scipy causes the test run to blow up from 6-7 minutes to 21 minutes (see https://travis-ci.org/megies/obspy/builds/13867509). We could/should probably address that by somehow exporting the compiled scipy (install with `pip install --egg`, then tar/gzip and unpack from that during pre-install?)