test travis on py2.6 too

travis: pip install obspy with `--no-deps` (all dependencies are
installed in "pre-install" section, avoids re-installing newer
scipy, see https://travis-ci.org/megies/obspy/jobs/13862500)

debug missing suds in travis

travis: properly install dependencies for py 2.6 test run

travis: try to fix installation of py 2.6 build dependencies of dependencies
Merge branch 'master' into travis_py2.6_rebased
Merge pull request #681 from megies/travis_py2.6_rebased

test travis on py2.6 too